Troubleshooting Idle Flutter Noise in a Mini Cooper | Engine Noise Fix

[DrChef]

Hello guys,

I am currently having an annoying issue with my Mini Cooper (1.5-liter 3-cylinder TwinPower Turbo).
When I leave it idling, above all when the air conditioning on, I can spot a strong fluttering noise clearly audible from the tailpipe.
It could be described as an impulsive noise, mainly high frequency content, counting about 20 pulses per second (my car revs at about 900rpm at idle).
I strongly doubt it is generated by the exhaust itself, even if I can hear it from the tailpipe. I think the exhaust is acting just an amplifier, getting this noise directly from the engine where it could be originated...
Did any of you experience this issue? Any idea/thought on what the root cause could be and how can I fix it?
Could it be calibration related (e.g. valve timing or other engine settings)?

Thank you

Cheers

 

[anorlunda]

How about a slipping belt?

 

[DrChef]

How about a slipping belt?

Mmm, that wouldn't occur at the main engine firing order, and anyway I couldn't explain why a slipping belt would generate a pulsation at the tailpipe.
I think it's more something to do with the combustion process or downstream the exhaust valves...

 

[Averagesupernova]

Sounds like something is loose and is vibrating at it's natural frequency. You will need to do more than listen to the tail pipe and hypothesize. Listen in other places. Use a wooden dowel with one end against your ear and the other pressed up against various places.

 

[xxChrisxx]

22Hz is firing frequency at 900rpm. AC increases engine load. So this could very well be normal. Really impossible to tell without actually hearing it.

 

[DrChef]

22Hz is firing frequency at 900rpm. AC increases engine load. So this could very well be normal. Really impossible to tell without actually hearing it.

Exactly, I think it's normal having pulsations at the tailpipe in every car because it's how a IC engine works, you will always have gas pulses at the firing frequency. The question is why some engines produce higher pulsations than others... what are the main things that can mostly contribute to this phenomenon?
I am interested to understand this from the physical point of view (of course).

 

[JBA]

By my calculation, at 900 rpm each cylinder fires at a rate of 450 pulses per minute x 3 cylinders = 1350 pulses per minute / 60 = 22.50 pulses per second.

 

[CWatters]

Did someone put a stone up there?

 

[DrChef]

By my calculation, at 900 rpm each cylinder fires at a rate of 450 pulses per minute x 3 cylinders = 1350 pulses per minute / 60 = 22.50 pulses per second.

Yes that's correct. That's why I said that this pulsation frequency would be expected. The problem is understanding why it results into a high frequency flutter noise.

 

[JBA]

It may be that it is the sudden discharge of the cylinder exhaust gas. I can't say specifically what the mechanism is; but, what you are experiencing doesn't seem to be much different that the flutter that is is heard in the extreme case from a single cylinder engine at low rpm's. By "high frequency" are referring to the pitch or sharpness of the sound you are hearing vs its pulse frequency?

 

[DrChef]

It may be that it is the sudden discharge of the cylinder exhaust gas. I can't say specifically what the mechanism is; but, what you are experiencing doesn't seem to be much different that the flutter that is is heard in the extreme case from a single cylinder engine at low rpm's. By "high frequency" are referring to the pitch or sharpness of the sound you are hearing vs its pulse frequency?

Thank you for your reply. High frequency I mean a broadband noise mainly above 500Hz. Usually in sporty cars you can hear this idle impulsiveness at low frequency, and that's normal and pleasant. The problem is that the high frequency content makes is sound like a tractor and it's not so nice to experience...