Tweeting sound indicates to me that the blower motor bearings were starting to go south. Would still work on high for a while until the bearings really fried, which is probably what happened.

I'd say you are in for a new fan unit. You can DIY if you are handy, gotta pull a lot of trim bits and the wiper motor and wiper assembly, it's a half day job the first time, a few hours the second time.

Rgds,
Chris W.
'95 E300D