I've been thinking about pulling the blower motor to inspect the bearings, etc. and lube, but am wondering whether the problem could be the controls. Soon after I bought the car it was making a hollow, knocking sound when the fan should be on high (high fan switch in, max hot or max cold), which would stop after awhile, but never any high fan. Low fan works fine. After a week and a thousand miles, the knocking sound no longer occurs. I have pulled the glove box and am now thinking that the sound might have been coming from the vicinity of the vacuum junction box. Could this be a control problem? Is there someway to force the blower motor into high?

Thanks for any help.

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Jim Wiggin
Columbus, OH
1989 300E

more than likely you have a bad fan speed regulator that is located underneath the fan motor under the wiper motor
paul