aspen is a good substrate but honestly I use paper towels...much easier clean up and personally I like it best. As for the squeak...it sounds like maybe your snake is getting ready to go into shed...especially if he is rubbing his head. Look for a blue phase...(sometimes it is easy to miss) but their eyes look a little blue/silver/milky prior to a shed. Make sure during this time you up the humidity to around 70%.

A good sign to look for with resp. infections is bubbles in the mouth, wheezing, open mouth breathing is a very bad sign.