knocking noise at idle

[wynvibe]

My 2006 vibe is a 1.8l, auto transmission. It has 111,000 kms on it. Recently it started making a loud knocking noise when warmed up and at idle with the car not moving, but in gear. If you slip it in neutral it will stop the noise usually, but sometimes it will restart the noise again in a minute or so even if in neutral. As soon as you put it in gear and start to move forward, it stops. It sounds like a loud engine knock, but the engine seems fine when running above idle. My mechanic has checked over the exhaust, and there seems to be no looseness there. He thinks it is the A/C pump gone bad. Anyone here ever had a similar problem or know what it might be? I hate to spend over $1000 on replacing the pump if that isn't the problem.

[09Vibe]

Since you had a mechanic inspect it I'd assume he is positive there is no internal engine issue (should not be with those k's on it). Only throwing a suggestion here because our previous 2.4L G6 did a similar thing where it sounded like a bearing at idle or bad counterbalancer knocking, kind of harmonic like you describe and would go away at times during idle or in gear. Turns out it was fuel pump pressure in the line, the line itself was rubber and close to the back firewall, it rubbed against it and the noise reverberated into the cabin in that manner. I could have sworn I was hearing a worn engine counterbalancer or something internal but thats all it was. Took a stethescope to find that and locate the fuel pulse noise. Perhaps you are hearing that too ! Good luck...

[joatmon]

a relatively easy thing to do is warm the engine up to where it makes the noise, then turn it off and remove the sepentine belt. Then start the engine up and run it for a short time (since the belt won't be turning the water pump) and see if the noise is still there. That would tell you if the knock is coming from the engine/tranny or from one of the belt driven components. Hopefully one of the less expensive and easier to replace belt driven components.A number of people have had issues with the belt tensioner making noises, but I can't remember what kind of noise they make when they start to go bad.