Distortion problem

Any ideas on isolating a distortion problem on piano music? Experience the same problem with two different sources into different inputs. I replaced the interconnects - no change. I have not heard the problem with other music, nor have I noticed it in the past. Problem is in both channels. Actually sounds like something is loose, but it does not happen often. Also does not seem directly related to volume.

I have Pioneer Elite receiver to a Rotel 1095 power amp to Thiel 1.6's. I know piano music can be a challenge, but the problem seems new and happens less than 5% of the time. Thanks for any ideas.
Check your Speakers drivers for loose screws..Just a shot in the dark........
My thought also, loose screws in speakers.
ALL cone speakers with screw need periodic tightening.
Just snug them up gradually going across and back etc.
Make the screws/bolts snug, not real tight.
You may need to replace the tweeters. Piano can be tough for tweeters.