I had the same problem with my 89 420sel, a chirping noise. I don't know how easy it is to remove the blower on the 300E, but it's a 5 minute job on the 420.
If you can remove it, put a couple of drops of automatic transmission fluid on both bearings, and let it seep in. Unless the bearings are really gone, it works like a charm and should last another 100,000 miles.
Make sure to plug it back in and test it for a bit, before you put everything back together! If it still chirps, add a couple more drops. I had to do it twice.
Hope it works,
Tom Garvin, Toronto Canada