Heater Fan makes a very loud noise. Want to know what to do to fix it.

Loud noise coming from the heater fan usually means the motor is about to go. This, of course, will mean no blowing of heat into the vehicle. Will need replaced soon. Other possibility is damaged or partially jammed blower motor wheel. Need to inspect to see which. Here's a vid : https://search.yahoo.com/search;_ylt=Atvf8xKqbAJllLrjdRCwsTibvZx4?p=how+to+replace+1994+mercedes+benz+e420+blower+motor&toggle=1&cop=mss&ei=UTF-8&fr=yfp-t-412&fp=1 If you want a shop estimate....try the estimator at the top of the page.