i have a 1999 trx400ex first four wheeler ive owned. ive had it about 2 months now and it is starting to make a whining noise in the top end. this is the second time its done it the first the oil was down to lower mark filled and all was fine noise went away. but went to start it again and noise is back. this is the first time ive started it since last week when it made the noise in the first place. is the noise the valves needing adjusting? or something a little more major? any help plz would be appreciated!