Not sure if you ever got this resolved.
However, I have possibly had the same issue (on a 12 month old PB). It appears to be a ribbon cable that has come loose and the cable+adhesive has gotten stuck to the side of the enclosure, so when you press they key it pulls the cable (which is now stuck to the wrong part of the enclosure) away from the enclosure resulting in a squishy sound and it feels awful, the note is also not triggered 100% of the time as you have to really push the key down...
I thought a piece of tape had gotten stuck at first and nearly pulled it out with tweezers. But, stopped as I noticed it was a cable.