it does it all the time, the faster the fan runs the worse it is
heater motor just started vibrating really bad on 2006 Ford E-250
There may be debris in the plenum that the blower is vibrating against. It may need cleaned out. OR, the blower motor itself is worn.
Look at the Fan , i have foungd that they crack . also have seen dirt /dust / debris, build up causing a ballance problem .