Sucky, rattly noise, low RPM, under load?

[Hopkins]

An unfamiliar noise has become apparent on my S1, I noticed it when putting the engine under load between about 2k and 3k rpm. It is slightly tinny and, at first, made me think of sucking foam through a straw so I thought about something on the intake, but it disappears after 3.5/4k rpm so now I think it might be more of a rattle catching some particular resonant frequency. I couldn't reproduce it with the bonnet up because there was no load. Perhaps there was a hint of it when I floored it from idle, but it blasts through the revs so quickly that there is no time for any further identification!

Any ideas?

[New Duke]

Can you get an audio recording of it? I'm happy to listen if you fancy a drive down to Colchester.

[Hopkins]

Thanks, very kind. I will try to get a recording, and I will let you know if I'll be down Colchester way soon!

[ChrisHolmes]

Check the heat shields around the cat, they're favourite for rattles

[dac69er]

My S1 had a similar noise. It's only very slight and doesn't effect how the car goes at all.

To me it sounds like mild detonation.

It's done it pretty much as long as I can remember. I've done trackdays and its never got any better or worse so I dont worry too much about it.

I'm not far from you in stowmarket

[warpc0il]

It would be interesting to run a tank of Shell V-Power (or similar) and see if it makes any difference.

In theory it shouldn't but there's plenty of evidence that it does on some people's 8s.

[dac69er]

It would be interesting to run a tank of Shell V-Power (or similar) and see if it makes any difference.

In theory it shouldn't but there's plenty of evidence that it does on some people's 8s.

I always use premium, different brands make no difference.
The only thing I remember being different is that I started using premix in with the petrol, but that could just be coincidence??

Would be interesting to know if the op puts premix in with the fuel too?

[Ollie]

I once ran non premium fuel and felt a significant difference. Always wondered if it was due to my premixing though as I was premixing a bit more than really needed (think my bottles hold about 200ml or something). But premium fuel just feels better so I just do that... The extra expense doesnt really bother me..

[Hopkins]

Hmmm, interesting suggestion... I've run 95 (usually Shell) with Idemitsu premix since I've had the car (from just over 40k to just over 70k miles over the last four years. My spark plugs are due for a change this service in about a month. By coincidence, I'm on empty at the moment so tomorrow I'll fill up with shell 97. The only maintenance I did recently to the car is an oil and filter change.

I'll report back after filling up. Thanks for the suggestions!

[350matt]

Also worth checking for a vacuum leak on the intake , on the lower / metal portion of the intake there are 2 little stubs that shoudl have a rubber cap over each
if one of these is missing you'd get this sort of noise

[Hopkins]

Sorry for the delay. So, I filled up with 97 and the noise was *almost* completely suppressed. I'm now just below half a tank, and the noise has slowly been creeping back to the point where it is obvious again. So I don't think it is the type of fuel. I have been able to record a sample of the noise which I have linked below - you can just make it out over the engine note.

https://www.dropbox.com/s/58vpp4gk58mrv ... e.mp3?dl=0

A couple of other things worth mentioning. I am hearing a hint of an exhaust leak on a cold start. I have also wondered recently whether I'm getting a tiny whiff of unburnt hydrocarbons inside the cabin. My sense of smell seems to be poor at the moment (eeek!) so I cannot be sure of this. It could relate to an exhaust leak, but also I started thinking about the fuel tank since a tank of fuel seems to damp the noise. I had the recall for the fuel tank done a year or two ago.

I'm due for a service and MOT in a month, so I will mention all these things.

350matt: I haven't yet checked the stubs which you have suggested. I intend to!

[dac69er]

i cant really hear anything odd tbh!

mine sounds like it has an exhaust leak on cold start too, but i dont smell any fumes other than around the back.
ive checked around the manifold as best i can too and there doesnt appear to be any evidence of a leak!

it does sound like yours has an issue very similar to mine, ive never managed to pin down what it is though. maybe its nothing and we are being too particular!?

[Honkytonk]

Really stupid suggestion, it couldn't be something fuel pump related could it? If it nearly disappears when the tank is full but is greater when it's emptier and under load, could that be the pump sucking (and rattling) on a drier tank?

[qwakers]

^^^ this thought occurred to me too

[30405]

Good god I've had a qwakers-level thought.

What a glorious day.

[Hopkins]

i cant really hear anything odd tbh!

It's a jangley metallic rattle, most audible between about 3 and 7 seconds, disappearing when I lift off at the end. It doesn't appear to be quite as loud in the recording as it is in the car. It is certainly new.

it does sound like yours has an issue very similar to mine, ive never managed to pin down what it is though. maybe its nothing and we are being too particular!?

I seem to be more prone to being "particular" than others, but clearly I do not believe that I am "too particular" . I suspect that this forum contains many such people!

Really stupid suggestion, it couldn't be something fuel pump related could it? If it nearly disappears when the tank is full but is greater when it's emptier and under load, could that be the pump sucking (and rattling) on a drier tank?

I would expect the pump itself to be indifferent to the level of fuel, until the fuel level is low enough to cause momentary starvation. I am only speaking from a point of "common sense", however, so I stand to being corrected!

[Ollie]

Funny I also had the same thought... Does this mean our rotors are in sync?

[qwakers]

the lower the fuel level gets the easier it is to hear any noise made by the pump.

[Hopkins]

the lower the fuel level gets the easier it is to hear any noise made by the pump.

Sounds plausible!

[Hopkins]

Although it does disappear at higher revs, even under load.

[qwakers]

logical, as the rest of the car will be making far more noise...

[Hopkins]

I'm convinced that it stops, not just that it is getting masked. I've been trying to listen carefully!

[Ainmhidh]

<guessing> If the pump's working harder (for higher rev demand) it might shift the noise frequency, or only be there at a certain demand load? </guessing>

[dac69er]

Do any of the various intake valves open around 3k rpm? Perhaps it is chattering or something? Although I would have thought there would be a bit of hysteresis to stop that.

[warpc0il]

Could it be the flow bypass on the fuel pump?

When the pump is shifting more than the engine demands then the excess flows back inside the tank.
If the fuel level is high then this is under the surface, and therefore quiet.
If the engine is under high load then very little fuel is "excess".

There should be a resistor mounted by the side of the Secondary Air pump that limits the fuel pump speed at low revs by some people link across this, either if it's been (or could be) the cause of an issue, or because they've read on FB that there's more power by deleting it...