Every now and then I'll pop in an old(er) tape that I haven't listened to in a while, and 2 or 3 songs in, it will start to make an awful grinding/squealing noise. This noise is audible through the speakers, not just coming from the deck itself. Its an almost "mechanical" noise, and is louder than the music itself. It stays steady and constant.
Tape squeal can happen for several separate or even some combined reasons... and you can sometimes hear it through the head and to the speakers as you described.
I've had this happen to some tapes, but usually only the ones that were quite beat up and not stored or handled properly at some point in their life.
Misalignment of the cassette tape within the case, maybe with an added dose of humidity (or worse moisture) and "long time no touch" can all lead to this. Tapes where you did lots of rewinding and fast forwarding in the past might be more prone to this than tapes that were played start to finish and put away in a case.
I would bet that the tape or tapes in question was/were probably not top shelf quality.. Maybe an inexpensive off brand, perhaps?
In some cases, doing a few rewinds and ff's to re-spool the tape helps. But not if there is actual physical damage to the magnetic tape itself.
Did you clean your head after playing a squeler tape? Was there lots of residue on the q-tip?
Personally, I'd chuck the tape out and re-record onto another one. I hope you are not dealing with irreplaceable recordings!
