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So my PT 4P has been an awesome horn for the past 6 years, but just recently sometimes one or two of the valves will make this really weird squeaky noise on their way back up. I've tried oiling them, fully cleaning it, changing oil, playing them almost completely dry, switching springs, but there is nothing I can do to make it go away. It kind of comes and goes - one day its there and will stay for a few days/weeks and then one day its gone. It doesn't seem to really affect the action at all. It's just a pain. Any ideas?

The best way to describe the squeak is almost like that sound you make when you inhale with a tight throat. Or those kids toys - the rods with the little thingy inside so that when you flip it upside down the little thingy moves and makes a funny noise...

thanks for any advice.

I cleaned out the bottom valve caps thoroughly. The sound actually lessens when I plug the bottom holes with my finger.

Or do you mean the bottoms of the actual pistons? I gave those a cleaning too, though not as thoroughly as the rest.... I'll check and see if that could be it.

A dental "water-pik" works very well for cleaning out those piston vent holes.
Also make sure that your felt washer (or whatever you use in its place) at the top of the piston is not covering the vent hole.

Thanks for the advice - I cleaned out the piston holes, but no gunk came out. They're perfectly clean.

Any other possible causes? Why would they make noise on and off? The horn has played without any of that for the last 5 years!
One of my valve guides is starting to wear. Could that do it?