My 92 F150 heater motor intermittently starts making a hight pitch squealing sound. I can sometimes play with the fan speed/ turning it off and on to get it to quit making the noise for a while. Sometimes it will stop making the noise for long periods of time and will then start again abruptly after hitting a bump.

Its driving me nuts!

The fan blows the same as it always has, it is just noisy. Would this be something stuck in the blower or the motor itself failing?

 

Most likely the bearings (bushings) failing. Fairly common problem on a blower motor when they get old.