Most likely the secondary air pump. This is located under the alternator on the V8 cars and only runs on start up until the engine is warm. Sounds like the bearings inside are going. I had the same thing on my 420SEL and had to replace it. It is not a cheap part. Some people just disconnect it, apparently it unplugs. As far as I know it only affects the emmissions system and having it off will not wreck anything else, but if you need to pass emmissions tests you will most likely need it in working order. However, I don't think we need to test cars over 20 years in Ontario unless you're up for emmissions testing this year you may not have to do it in the future.

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Jason Priest
1999 E430
1995 E420 - retired
1986 420SEL - retired