chain harmonics

[zootpatutie]

Just put on a Kenda 270. Went for a ride and noticed a cyclical noise which sounded like chain/sprocket noise in a rythmic hum, coming from underneath. Checked and readjusted the chain tension with some improvement. Rode home and discovered a pine cone resting on the chain, but removing it didn't change things. Can hear it at slow and intermediate speeds, gone at higher speeds. Can also see where there is some light contact between the tire and the muffler (looks like just the new tire strings that stick out from the tread lugs). Chain measures short of 12 3/4 inches at the 20 link distance, can see no lost o-rings or other obvious wear, and have no oil leakage from the front sprocket. Tire is seated all around and spokes seem tight to the touch. Am I looking at an emminent chain failure, is the harmonics part of having bigger tread patterns, is it possible to have the rear sprocket incorrectly seated, could rear bearings be worn? Any help appreciated. Todd A9 (sometimes with too much rythm)