This is concerning me. When I am on the highway at around 3000 rpm, I hear squealing and it seems to be coming from the front passenger side of the engine. Around 70mph, I hear the squeal. If I let up on the gas, it stops. If I turn on the cruise and am going down a hill at 3000rpm, even a little one, the squeal stops, but driving flat or on an incline, it squeals.

Do you think its the belts or something much, much more expensive?

I had a very similar noise with my 85 300D. I posted so many questions trying to find out what it could be. (Do a search of my user name) Does it happen right after an upshift? First, I thought it was the turbo. Then, I thought it was an exhaust leak in the flex tube down pipe. Finally, after reading online and check, and reading online again, I decided it must be the ARV (air recirculation valve) on the turbo. Dropped a BB into the vacuum line to it, and voila, no more squeal!
Do your own research to be sure. But this silenced my noise.

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'85 300D 190K miles
running on B100 Biodiesel since 156K, SCRATCH THAT! I can only find Biodiesel at 1 location in Houston! (not very convenient)

If it is not belts... the usual way to check for something which might be a whistle or squeal caused by air being sucked into something... is to apply oil to the gaskets or edges where it might be happening... can often pinpoint the little hole...

Hi flyin4now...yes on upshifts around 3000rpm, did you experience the same squeal on the highway around 3000rpm and did it go away when you were going down an incline?

Hi flyin4now...yes on upshifts around 3000rpm, did you experience the same squeal on the highway around 3000rpm and did it go away when you were going down an incline? Also, what is the best place to find instructions for the procedure you spoke of?