Over the last several months, my engine/exhaust has been gettiing progressively buzzier. Could this be caused by a leak somewhere in the exhaust? If I get the exhaust checked out and no leaks are found, anyone have any idea what could be causing this? The engine is presently running well otherwise, with very good mileage (just got back from a trip up north, almost all highway and got close to 600 KM (372 miles) on a full tank, average speed of 120KM (75 mph).

I've got a K&N drop-in filter which was cleaned and re-oiled a while back and I've tried driving around with the stock paper filter but no difference in noise.

By the way, the buzzing (not really bees-in-a-can bad - yet) is pretty constant across the RPM band.