Piston Slap / Rod Knock ?

Your apprehension may be very correct.  I once noticed a slight worrying sound my Sube was making...sure enough...within a couple thousand miles the timing assembly came loose on the highway.  I managed to limp it the final 5 miles to the shop.  The racket it made was absolutely awful...but it got fixed and now it runs perfect.

Also, check your exhaust for leaks...even a small crack can make a really big difference in engine performance (as I recently discovered).  You may not have noticed it because the corrosion can be slow and subtle.

--Damien