UNKNOWN ENGINE or EXHAUST TAPPING SOUND?

I have a '95 5.0 with 87K miles all stock motor except for full length headers. This weekend I noticed a clapping/tapping sound coming from the passenger side around the alternator/ oil filler location.
The clapping sound can be heard at idle and does get faster as I rev the motor.
I did check the oil and it was toward the bottom of hatch area of dipstick. I do leak very little oil by the oil drain plug (plug may be stripped or washer worn) but the leak is very very little.
I did add some oil anyway and the noise lessened a bit but still very noticeable.
The sound reminds me of a lawn mower the way the motor "claps" away almost like a tractor would sound.
I only drive the car on weekends and the last tuneup was 4 yrs ago about 8000 miles ago.
Is it my valves sticking or maybe my exhaust header bolts backed away? I had headers installed 3 yrs ago.
Any suggestions, I hope this problem is not a serious one - low on cash.

Thanks for all your input guys. I'll check it out this weekend and I'm starting with the headers and hopefully that's it.
I'll keep all posted. Thanks.

You are right, the header bolts were loose like teeth.
I tighten them up (the ones I can get to) and that helped a lot, but I still hear that clapping sound but a lot less. I think the header gaskets are no good. They're BBK, I thought they were made with reinforced wire.
I will buy Mr. Gasket copper gaskets, hope that will fix it