86 300E - Bad Blower Motor?

[Phalcon51]

A couple of months ago, my blower fan started to make a muted noise (sort of a rapid tunk-tunk-tunk-tunk-tunk) whenever the fan was set on low speed. At high speed, it just blends into a kind of low-volume drone. It sounds like the squirrel-cage is hitting something every time around. Lately, it will run when the car has been sitting overnight or for several hours, but once I run it for 10 or 15 min. and shut it off, it won't come back on again, as if it's binding from heat or something. Is this a typical failure mode for a blower motor? Do the squirrel cages themselves, being 19 year-old plastic by now, ever self-destruct? Seems I've read posts complaining of screeching noises from bad bearings/bushings, but I haven't come across a description of this problem.

Is the fan/motor relatively easy to extract? Approximately how much time might it take? Can anyone point me to a how-to or DIY article. I don't want to order parts until I know exactly what the problem is.

Thanks