Strange knock noise

#1

So I noticed a weird noise this weekend. It almost sounds like someone tapping their knuckles on the body of car. None of us present at MidOhio race could figure out what is causing it. Here’s a recording of it, pay no attention to video portion as I’m not trying to show anything, just using it for the audio recording of a knock sound that is only audible in the cabin.
http://youtu.be/OdKQA0Z8XjI
Frequency of the knock is engine speed dependent, video was taken at 2000rpm where it seems the easiest to hear. No change if the clutch is in or out. Cannot be heard from outside as the normal valvetrain noise is louder. You can feel the knock if your hand is on the cowl steel just below the windshield. You cannot feel the knock with a hand on the intake manifold or on the transmission from the bottom side but this may just be due to it being washed out with other normal engine vibrations. Rigid poly engine mounts.

I have no idea how long this has been there but I noticed it after the morning warm up when I was slightly raising the idle (for oil pressure) while the car was cooling down. That’s not something I normally do but I was watching how quickly oil temp decreases during cool down because I just added that sensor. Afterwards noticing this, I ran qualifying with one of my best times ever and the race including standing start with no ill effects. Noise is still the same as it was when I first noticed it.

Any ideas? I’m stumped.

#2

The rate of the knocking is oddly slow. Classic knock types:

Rod: Warm engine.

Piston slap: Cold engine.

Things you might try:

  1. Confirm that the sound does not occur when engine is cold. This reinforces rod knock idea and makes transmission related causes less likely.

  2. Pull each spark plug in turn and see if sound goes away with a certain plug removed. That could reinforce idea that the engine is the problem and isolate a piston.

See how that goes before we move on to harder tests.

#3

Check your fan belt. CB

#4

Ranger: does it warm or cold, makes no difference, no change in the sound either. I can try the plug thing next time I’m working on it (maybe Friday). I know what piston slap sounds like and it’s not that. Not sure I’ve experienced rod knock in person but the frequency of the noise does not match the frequency of the engine (2000rpm = 33.3rev/second) or any firing pulse of a single cylinder.

Chuck: what fan? Electric pusher only. Or do you just mean the accessory drive system in general?

#5

[quote=“jlucas” post=67020]Ranger: does it warm or cold, makes no difference, no change in the sound either. I can try the plug thing next time I’m working on it (maybe Friday). I know what piston slap sounds like and it’s not that. Not sure I’ve experienced rod knock in person but the frequency of the noise does not match the frequency of the engine (2000rpm = 33.3rev/second) or any firing pulse of a single cylinder.

Chuck: what fan? Electric pusher only. Or do you just mean the accessory drive system in general?[/quote]

The freq issue is good news. The problem may well not be serious.

Re. Chuck and the accessory belt. You guys are talking about the same thing. I think everyone calls that a fan belt, even if that’s not really correct.

#6

Well, it is a stinkin fan belt because it originally drove the engine fan. Does sound like a section of the belt is missing. cb

#7

One other piece of info is we just replaced the steering rack. At the track, after noticing the noise, I checked the engine mounts/subframe/and rack mounts torque which were all fine.

I guess one thing I should check is if the replacement rack could be close enough to hit something when everything is vibrating.

#8

That’s a strange one Jeremy.

You said it changes based on engine speed though right?

#9

Ok so I think our knock noise was the alternator. The lower mounting bolts was not tight, so it’s probably been that way all this season since we had everything off over winter to do the timing belt and probably toasted the internal bearing.

With the belt removed the noise is gone. The crank pulley and water pump are also that belt so there is a small chance they are the culprit but when you spin the alternator by hand (off of the car) the vibration feels similar to to what could be felt in the body. When we pull the head to fix the overheat damage I was already planning on doing the WP (already have the part) and I’ll check out pulley/vibration damper at the same time.

#10

Confirmed. Rebuilt alternator = no more knock. Weird but true.