The 1963 vintage (D-type, I think - the earlier one) overdrive on my car makes a high pitched squealing, noticable mostly on the highway. Not a something is seriously wrong noise, but loud enough that it gets irritating enough on long trips that I may turn the overdrive off and drive in forth/direct just to make it go away.

Is this normal? What I am wondering, is this just the nature of the beast, or a warning that something is getting ready to let go. The car is an open car with minimal sound proofing, so it is possible I am hearing something normal, that is just not that noticable in other cars.

I have the car apart now, so if there is something I can do about it, now would be a good time to do it.