I have this chirp that sounds like a chipmunk coming from my engine. It happens when I'm coasting along from 50 to 30 mph. It will cut out as soon as I hit the accelerator though. I can't figure out what it is. The car is an 85 300SD, anyone have any ideas what may be causing it?

Does the frequency decrease with the decrease in engine rpm ? Does the volume decrease as you go slower ?

No, the frequency stays the same, unless i give it a little power then the sound will get a little louder then all of a sudden stop with the addition of cruising power. It's when the engine is idling btw. 50-30 mph, but the sound will dissappear below this speed.

It's not the wastegate opening/closing is it?

__________________
1985 300D Turbo ~225k
2000 F350 (Powerstroke) 4X4, SWB, CC, SRW, 6spd ~148k
1999 International 4900, DT466e (250hp/660 ft/lbs), Allison MD3060 ~73k

No, I don't think so. It sounds more like a belt or squeaky bearings. I cant figure it out cause it doesn't happen when the car is sitting still only when moving.

Could be your enigine nailing, or your timing chain rattling when you hit the gas.. how many miles are on this beast?

Curious deal, but you've got some good clues. 1. car must be at speed (30-50) 2. engine must be at idle (below 1k rpm?) 3. Acceleration or braking eliminates noise

Doesn't sound like wheel bearings - no variability in frequency of chirp with change in speed. Doesn't sound like engine bearings or belt, again because there is no change in frequency of chirp with change in RPM's. But, the combination of wheels at speed and engine at idle brings on the noise. Some basic checks will eliminate some possibilities and help get your head moving in the right direction. For instance, check out the brake pads/rotors for wear. Check vac. hoses, especially the vac. evacuation hose that runs from the vac. pump to the air intake. Check your belts for any burn or glazing. If you haven't found anything by the time all of this is done your mind will already have you focussed on where to go next.

Keep us posted.

The sound is coming from the engine. It sounds more like a chirp of a grasshopper. It is not very loud but just enough to detect. I just had the waterpump replaced and it stopped after that for about 50 miles and then returned. The car has 219,000mi.

Replace your belts when you did the water pump?

try revving the engine just a tad above idle. When your going 30-50mph your engine isnt at the same rpm as when your at a dead stop, it's like coasting a stick in 5th gear, in other words, it doesn't completely disengage when coasting
R

__________________
83 300SD.......sold
96 integra SE....sold
99 a4 quattro....sold
2001 IS300.......sold
2002 330i.........current.
2004 highlander limited....current.

It's probably H20 pump related, then. If it spent 50 miles not operating and now it does... dubious goings on there.

Also the workings within the H20 pump are going to be reacting to coolant flow which is determined by temperature. Wheel speed and RPM's are not the direct determinants of the coolant flow.

Perhaps the pump was not correctly aligned when installed? Maybe the belts were not properly tightened?