At very low RPMs (such as when setting minimum idle speed, 500 rpm, and occasionally at idle in drive, 600-650 rpm), there is a buzzing noise coming from my engine bay. I have been unable to locate it, although I haven't tried the screwdriver-stethoscope method (I'll try it next time I'm working on the car). I've heard before that the alternator makes a buzzing noise when it's on its way out... true? Any other likely causes?

 

EJ,

At very low RPMs, the transmission will make a sound like you describe due to the low flow of the trans oil pump. This was the subject of a TSB on TH700R4s and is not a major concern. The TSB stated the problem was most noticable at low RPM in gear, but could be present in neutral/park.

The oil flow is barely enough to maintain adequate pressure. What you might be hearing is the oil pressure relief seating and unseating very rapidly, with every pulse of the pump as the vanes pass the discharge port.

If the noise doesn't sound as if it's coming from the transmission area, you should revert to the stethoscope plan and start looking in the accessory drive area, like the power steering pump and alternator.

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Later,
Vader
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"Make Me Bad"
Adobe Acrobat Reader 4.0

 

My car makes a buzzing noise alos. I think it's my injectors. Is your car TBI like mine?

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92 RS (Rice Sucks) 305 TBI 5 speed T-tops AC stock 16 inch aluminum wheels
14X3 open element, 3.73 gears, posi, dual 2 inch bendable exhaust, no mufflers or cats (kinda loud), 160 thermostat, relocated IAT, weight loss, lots-o-free mods and one hell of a crazy driver.

"Friends don't let friends drive Mustangs."

 

RSS, yeah I've got TBI but it's not the injectors. My injectors do make noise but this is a lower pitched and slower buzzing, almost like a rattling.

Vader I think I'll give the stethoscope idea a shot. I tried putting the car in neutral and drive and it made the noise both gears even when the RPMs differed significantly (200 RPM?). It seems to be becoming more prominent at higher RPMs (I heard it once a while back when setting minimum idle (450 rpm), now I've heard it a few times in OD (600 rpm) and today I heard it in N (700 RPM)). Man I really hope my alternator's not on it's way out...

Also I checked tranny fluid level, it's still good, just like it has been for several months now.