rattling sound under valve case cover!!SOS

hi everyone, my 91 explorer makes a rattling sound that comes from the valve case, it last for 20 minutes and then it quits. Always when i started cold, when warm it stops. any clue of what it can be? Of course i check it for oil every now and then.

Any help will be appreciated...

Thanks

Hmm- may be the valves or lifters are wearing out. How many miles are on it? My 94 has a little clatter for a few minutes till it warms up, say 5 minutes- and then it stops. What oil are you using in it currently?? If you are not using 5w30 use it and don not use a FRAM oil filter because of its poor design.

... my 91 explorer makes a rattling sound that comes from the valve case, it last for 20 minutes and then it quits...
I agree with molehole.
You will need to adjust the valve clearance. If you have the factory service manual from Helm, Inc. you can do this yourself. The concept is simple, but the procedure is labor intensive.
I've been searching for the illustrations online, but I haven't found anything.