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Running a 2011 MacBook Pro with Snow Leopard installed, Logic Pro 9. I used to run MachFive2 on Logic 8 and had persistent playback and exporting issues, especially with piano sounds. Now that I'm on 9, they're even worse. I've changed buffer size, exporting settings, quit all other applications, reset p-ram, and even created a session specifically devoted to JUST that single piano track, running no other plug-ins and with no other regions. The clicks still come at regular (I'm talking at least one every two seconds) intervals.

Normally I'd chalk up these issues to the Logic update, except that they were persistent in Logic 8 as well.

Help, please.

M5 2 was ok with L8 here.
No issues with M5 3 and L9. (7200 rpm internal drive for OS/apps; 7200 rpm FW/USB external drive for sound banks or internal drive on MacPro, 8GB RAM in MBP, 12 GB in MP). Runs smooth in 10.7.4 and 10.8

It would be helpful to know:
Amount of RAM?
Sound Banks on system/application hard drive or on a separate drive?
Speed of hard drives: 5400 rpm or 7200?
Using M5 effects or those available in Logic?
Type of audio interface: FW or USB? Brand?

Often clicks are caused by a CPU overload, slow drives when streaming samples or bad ram.
Funny it is only from one sound, the piano. .Does it have a large file? Try a different large file and see what happens.