Upgraded recoil springs do increase wear on the sears - you do know that, right? Might not be the case with TM's, at least I understand that its not the case with TM Hi-Capa's.
As to a valve knocker, I was thinking something more along the lines of the spring that is attached to the knocker, or that the knocker itself has a fracture... But peering through the mag well won't show either of those. However, since the knocker is move forward with the hammer... I dunno.
As to a hiss of leaky seals? Nope, you won't always hear it - take for example, one of my KWA USP mags. If all is dead silent and you hold the mag up to your ear, I can barely make out a very, very slight hiss... sometimes. However, I can tell that something is leaking because leaving that mag filled up with gas for a week and a half will result in an empty mag. In your case, you might feel the results of it, like a rapid drop in power... but it really depends which seals are not doing their job. The seals I'm thinking about on yours are the ones at the top of the mag - I've noticed that these seals on my Socom Gear/WE original mags (more wear and tear on everything original on there than there should be for its age - but that's a result of buying a used gun) feel like... well, dying rubber, in comparison to my newer mags that have firm seals that actually form a complete seal with the loading nozzle.
Bring it to a gun doc. I still stick with that advice. There comes a point when countless hours of fiddling/experimentation is just not worth the time, and paying someone else with leagues of experience to take a look at your GBB for a few minutes just makes sense. At least, if its something more serious, they can fix it.