I noticed something yesterday when my wife & I headed up north for lunch and to enjoy the weather. After climbing to cruise alt. - 4500' for this jaunt - I noticed what seemed to be a slight miss in the engine. Nothing extreme, just not the usual smooth running I've grown accustomed to. I seemed to be runing fine during climb, but when I leveled off and started to lean the mixture, I noticed it. Going to full rich didn't change anything. Fuel pump on - no difference. EGT was fine, RPM stayed constant. There were no other indications anything was amiss. It was annualled only about 10-15 hours ago and I know the plugs were cleaned and timing set. Run-up was fine with no problems with the mag checks.. I was thinking of just puling the plugs to check if 1 may have gotten fouled, but the engine has always ran fine . Any thoughts out there?