My neighbor's 2003 Camry (2.4L) had the same issue. The cause was 2-fold, first there was sludge built up under the valve cover and the 2nd needing a head gasket and engine block because the head bolt threads pulled out of the block. I would keep a close eye on your oil level, pull the valve cover and check for sludge, inspect for leaks around the head gasket area, and most definately check for loose head bolts as the threads may be stripped in the block.