If you're getting a screeching noise under load and you seem to lose turbo boost, be looking for a leak on the intake manifold or some of the plumbing associated with it. Someone here recently was complaining of a loud whistling noise and low boost and had a cracked ALDA line. The turbo should be your very last subject. They're bathed in oil and have bronze bearings, so seizure is extremely rare.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]