<VV> What's squeaking?

[mhicks130 at cox.net]

My '65 has developed a "squeak" in the right rear.  At first I thought it was rotational and suspected the brake or bearing or even the heater hose rubbing on the drive shaft.  Well I did thourough fault isolating today.  I jacked the car up and put it in gear and got no squeaking.  I then pulled the wheel and drum - no squeaking.  I then took the drive shaft off - no squeaking. I had my wife sit in the car while I looked under it and there's plenty of clearance for the heater hose.  I put it all back together and took a test drive - no squeaking, until I had almost returned home (approximately 5 mile round trip) and I heard squeaking.  My wife was in the front yard and she heard it and said it sounds like a "squeaky mattress spring" sound.  I had assumed it was rotational but I now think it is up and down related.  So who's the likely culprit?  I know it's hard to diagnose such things over the internets but I thought I'd try anyway.

mike