Engine whine or whistling low RPM initial startup

[JoeFFG9]

It seems like for the past few months we've heard this whine or whistling, usually after flight and after refueling with a warmer engine. It can be heard at lower rpm. I noticed it the last time after startup that it didn't immediately start the whine until a few seconds after startup. Checked vacuum pump and it is not the cause. Can't seem to locate where it's coming from. No unusual engine performance and the CHTs are normal across the range as far as we can determine.  Here is a recent video.  You can hear it better when the engine is lower rpm. Better to listen with headphones on. Any ideas?  

 

[Slick Nick]

Almost sounds like an alternator bearing or something, but doesn't seem to change much with RPM?

[Rwsavory]

Two guesses: induction or exhaust manifold leak, alternator. 

[Jim F]

you say after a flight and after fueling.  Does that mean you hear it after landing and before fueling and after fueling?

Kind of hard to hear or maybe it's my ears.  Un-cowl and try to get it to repeat.   You can close in on where a noise is coming from by using a stethoscope or long screw drive with the handle pressed on your ear.  You're working around a moving propeller so be careful and you need someone on the breaks.  Place the blade on a what you think is making the noise.     

The most common noise after starting is usually a hung Bendix drive on the starter.   You can start the engine and if you hear the noise shut it down.  Look at the Bendix if it's still out (Fwd) its sticking.  Clean with contact cleaner and load is up with silicone spary (zero hydrocarbons - oils), this might take multiple application

[JoeFFG9]

14 minutes ago, Jim F said:

you say after a flight and after fueling.  Does that mean you hear it after landing and before fueling and after fueling?

Well. Now it seems like it happens all the time. It use to be only during startup after flight with a hot engine. 

[JoeFFG9]

1 hour ago, Rwsavory said:

Two guesses: induction or exhaust manifold leak, alternator. 

Yea. We are learning toward induction or exhaust. It seems like it was at one time, only after a flight and a hot engine but now it occurs during cold startup.

[Jim F]

All the time is good.  Look at the intake tubes and touch them.  Most of the time a leaking intake pipe will show blue stains around it even if injected.  Pay attention to the metal tube that are swaged into the oil sump, look for stains or run and spray WD40 where the tube goes into the oil sump, at idle the rpm will increase. caution (moving prop)  Look at all exhaust studs.  Pressurize the intake and exhaust with the pressure side of a clean shop vac spray soapy water.   Move from easy to hard

[Kelpro999]

When I’m looking for sounds like that I use a three-quarter inch 3 foot long piece of A/C hose. It helps put my ear in places I can’t get to.
 You can also remove the alternator/generator belt, tie it  back and retest.

[JoeFFG9]

Here are the engine monitor images from today. I'm noticing that the #2 cylinder is running higher EGT than the other cylinders and lower CHT. Could this be a sign of where the leak is coming from?  

[Will.iam]

Could be but depends do you run rop or lop? If lop is the throttle wide open? If not what MP are you running?  

[JoeFFG9]

1 hour ago, Will.iam said:

Could be but depends do you run rop or lop? If lop is the throttle wide open? If not what MP are you running?  

I was running around 24 MP with mixture leaned to before running rough. About 8.5 gph.

[Will.iam]

So when you sweep to lean is number 2 cylinder the first one to peak or is number 2 the last one to peak?