Did you check all the adjustment nuts when you checked the clearances? Sounds like on of them might be coming loose and that ain't good! When was the last time they were adjusted also.... like samiam4 said it does sound like they could be out of adjustment, I would check them all again just to be safe.

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1985 300D Turbo ~225k
2000 F350 (Powerstroke) 4X4, SWB, CC, SRW, 6spd ~148k
1999 International 4900, DT466e (250hp/660 ft/lbs), Allison MD3060 ~73k