It is probably the motor itself it sounds like u have a bearing going bad you can view my tutorial on this at the following link it will help u test this problem http://www.benzworld.org/forums/w201-190e-d-class/1358590-troubleshooting-Mercedes-Benz-190-e-d-210-tempmatic-climate-control-system.html#post2716281
either the blower control switch or the blower motoer needs replacing
Blower motor resistor is bad. It is a little circuit board with resistors on it that changes voltage to the blower motor. When it goes bad, you get full speed only.
Blower switch and connection had a melt down.
The blower motor resistor on a Mercedes 300e is located beneath the blower motor on the Mercedes 300e, if ever it needs replacing.
where is the blower location the Mercedes Benz 300 d
The blower motor on a Mercedes Benz 300e is changed by disconnecting the wiring harness and unbolting the blower from its housing. The motor is then lifted away and a new blower motor connected.
385 euros @ mercedes main dealer.
From what I know the blower brushes are worn down. Easy fix just do a search for "replacing blower brushes on 85 Mercedes 300D ZaneRen
it sounds like you need to replace the blower resister under the glove box
need to locate the blower motor on Mercedes 300E 3.0 motor
If the blower runs on high it will get hot. If the blower is wearing out this will cause it to get to hot and blow.
To replace the blower motor on a C230 compressor of a Mercedes Benz, remove the bolts holding the motor in place. It is next to the blower, under the passenger side of the dashboard. Replace with new and reinstall the bolts.