Mystery noise goes away with AC off or when clutch is pushed in?

SPUDSMCZ

MEMBER
1999 FORD RANGER
4 CYL
2WD
MANUAL
185,000 MILES

Hello, I have a booger of a problem that I cannot figure out. There is a knocking noise that I hear from my pickup and have no idea where it's coming from; it sounds louder from underneath but I can't pinpoint it. The noise goes away when I turn off the A/C, however the noise "also" goes away when I push the clutch in. Now I thought that maybe it was related to how the WOT switch cuts off the compressor but I didn't think any of that was connected to the clutch, just the safety/interlock switch. I am so lost and this noise is driving me bananas! Please help! I have a recording of it, if that would help.

3 Replies

KHLOW2008

MECHANIC
41,814 POSTS

Hi spudsmcz,

Thank you for the donation.

Noise are never easy to detect even with the physical vehicle so we might not be able to help you.

Anyway let us give it a try.

Is the idling speed similar when A/C is turned "on" or "off"? Does the noise disappear when idling is increased slightly?

Since this is a manual, there is a possibility it is the transmission clattering at idle. Using a stethoscope or rubber hose to touch onto the trans might allow you to verify if the noise is from within.

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Saturday, July 11th, 2009 AT 12:17 PM

SPUDSMCZ

MEMBER
2 POSTS

Thank you for your response, here are the answers to your questions:

Is the idling speed similar when A.C is turned "on" or "off"?

A) Yes, I cannot be sure how close though because I don't have a tachometer, however I don't recall a rise in RPM.

Does the noise disappear when idling is increased slightly?

A) It seems to fade away when idling is increased slightly, though if I listen closely I can hear it a little, but just faintly. This is idling only increased by a hair.

My big question is, why does the clutch being pushed in make this noise go away?

I have pretty much resigned myself to this problem being the compressor. With my wife this evening, I had her turn on/off the A/C and push in the clutch. When the A/C was turned off/outside air the compressor quit (and I noticed it was vibrating) and the sound went away completely.

When she pushed the clutch in, the compressor kept spinning however the sound also went away completely. I am really confused as to why the clutch is making this noise go away, unless the transmission makes the same rattle and turning the A/C off makes it go away, too! Which would be just as confusing! (To me anyways).

Any ideas why (clarified question) pushing the clutch in makes the compressor stop knocking?

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Sunday, July 12th, 2009 AT 10:24 PM

KHLOW2008

MECHANIC
41,814 POSTS

When engine is running, the clutch shaft, counter shaft and gears are rotating. This would create a load on the engine. When the clutch is depressed, the clutch sfat stops which would stop all the gears and counter shaft from spinning. This would lighten the load and reduce or eliminate the noise.

As I mentioned, it could most probably be the gears rattling but not too sure as you can't confirm the source of the noise.

Other than the gears, it could be some other items such as exhaust or heat shields and when load is reduced, the noise goes away.

When rattling starts, try using a glove hand to push against rear exhaust to see if the noise changes or goes away. This is to confirm if it is related to the exhaust.

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Monday, July 13th, 2009 AT 9:48 AM