Hello All,

I have a 2005 G35x with about 160K miles on it. Recently a nasty knocking sound was coming from the front of the engine. Before that it sounded like a droning sound but now has turned into a knock.

The PS/Alternator belt eventually broke and then droning sound turned into a knock. But...the sound doesn't happen when the PS/Alternator belt is not on the engine! WTF?

I figured the PS pump failed and caused the belt to shred so I replaced that and the knock is still there. So I replaced the alternator and all the pulleys for that belt. Knock is still there. When I removed the belt and start the car there is no knock.

I have no idea what it is at this point. I was thinking it could be the water pump but with the belt off there is no knock and the belt doesn't drive the water pump...it is chain driven.

Now I am thinking that the issue is with the timing chain or the water pump. The knock sounds like it is coming from that general area. But I still don't get it why removing the PS/Alternator belt stops the knock. The engine runs pretty smoothly but does surge or hesitate at times with or without the belt on.

This is my daughter's car and she has said before all this happened that the car would stall randomly but not very often and sometimes would shudder but it was random and not very often.

So any ideas on what this could be?

Could the extra pressure on the crank pulley when the PS/Alternator belt is on be causing some kind of issue with the crank? But that doesn't make sense because the sound is coming from the top part of the engine. Water pump and/or timing chain tensioner possibly?

Thanks in advance!

Videos of the knock with belt on and no knock with belt off...

 

Don't know why these didn't show up in my main post...

 

So I took off the timing chain tensioner cover. Looks ok but when I rotated the crank manually I could see the tensioner move in and out. I don't think it is supposed to do that right? Supposed to stay in place to keep tension I would guess...

Also scanned the car and no report of misfires, etc. So that is a good sign.