It's impossible for me to diagnose the problem with 100% certainty, but it sounds as if there is too much clearance between the bakelite fan assembly and the main drum drive pulley. This will cause the fan to work its way back, hit the back of the fan housing and then bounce forward again. Have you every worked on an appliance such as this before? This adjustment is a relatively simple one given the drama that CAN ensue with an old HOH dryer.
Basically, all you need to do is remove the back of the dryer, loosen the retaining screw on the drum drive pulley and move it forward about an 1/8" for starters. That may not be enough and you may need to move it in a bit further depending upon how far it's drifted. I've done this on two HOH dryers that have exhibited similar issues and it usually solves the issue.
On second thought, it may also be a simple issue of the cabinet not being perfectly alligned. HOH dryers are NOTORIOUS for not being happy unless they are perfectly level and squared. In that case, loosen the screws on the back (there are about 16 of them). You don't need to remove the back, just loosen the screws so that the back has some "play" in it. Place the dryer in its operating position and level by ajusting the feet using a proper level. Once it is perfectly level, retighen the back screws and this problem may might just go away. If it doesn't, contact me directly.
