The pitch is always the same. Same peep each time - sometimes the squeal/whistle will last one second but at times I've counted up to 5 seconds when going up a steep hill. It's always the same pitch and I've heard it at 30 mph and 60 mph always the same pitch. I've never heard the noise when decelerating below 30 mph or at a red light. I drove it 2500 miles in 3 days, air conditioning off and on etc (still heard it) and heard it quite often in the mountains. The only time I can get it to start is to get the engine fully up to temperature and up to 3000 rpm and then lift off the throttle. Most times it will squeal/whistle for a brief second. When going up a steep hill for a half mile or so, it will start to whistle/squeal for longer time periods even when I haven't lifted off the throttle. You can hear it in the cab of the car but have a hard time hearing it under the hood due to the diesel, but you can get it to squeal when revvving the engine and letting go. I've changed all the belts and retightened them again. No impact - same noise. Changed radiator cap - same noise. Someone said to check the overboost sensing switch hose from the back of the intake manifold to the drivers side firewall mounted switch. I pulled the line but I can blow backwards (mouth method) through the hose and into the intake manifold so the line doesn't appear plugged. It's actuallly fairly clear looking with only a minor amount of soot near the overboost switch. Still stumped.
Harold
1985 300SD 5 cylinder turbo diesel 92000 miles