97 chevy lumina auto, v6, 3.1 engine tapping power loss misfire when warm

97 chevy lumina auto, v6, 3.1 engine
first off the car has routine oil changes has 81,000 miles
last change was done October 08 a month later it developes this missfire, loss of power, and a loud tapping noise from the engine compartment.
thought it might be the lifters or valves we took the valve cover off when it was hot and turned the engine and everything appears fine: lifter, springs and valves appear to be working fine. WHen the # 3 is down it makes a loud chatter, there is some sludge in there, oil appears to be watery, clear not dark or anything.
not milky in texture, no oil residue in the water..so it does not appears there is a crack in the block or blown gasket.
Can any one tell me what could be causing this tapping noise.
is this the piston slap everyone talks about?
next chance I get I will try to clip a video of the noise and post
Cams?
rockers?
valves?
maybe injectors are clogged?
if everything else seems to be working fine.
Does anyone know if the sparks plugs can make a difference on this make model?
I have considered changeing spark plugs thinking maybe one cracked.. not likely
which would you use to try and resolve this issue? : STP, EOS (Engine Oil Supplement)
or sea foam?
a mechanic over a year ago told me there was a light on and he would take care of it and disconnected it.. turns out I was told that it was the "check coolant soon" light, the temp gauge has not run high.. but could I still be leaking water into the engine even if the oil isnt pudding like? or the radiator does not have oil in it?

The car is not driven but 5 miles a week and I dont want to loose the engine over this.. so anyone have a past issue or know of known cases where this is resolvable please post back.
money is an issue on a set income and no money to buy a new car and
well little to put to repair if it is a cam issue
ty

From here noises are impossible to diagnose, have you had it scanned to pinpoint which cylinder is missing, are you loosing any coolant? What does "when the #3 is down", mean?

well I am impressed the spark plugs took care of the loss of power and the missfire and the seafoam took care of the rest !!!
I have never used this product but I can and will say that I am impressed from all I have heard it stands up to what they say it will do.
The plugs were original and they had nothing left to them ..
I ran out of time to do an oil change and decided to just add the seafoam to the crank case and the fuel, just enough to condition them not the extra to flush and change the oil I told her to take it and run it for a few weeks adn then we will do a flush and oil change .
I did take a snap of the noise before and after if I can figure out how to upload them and where so that I can place a link to hear them people will be amazed at the difference...man what a difference..

http://i262.photobucket.com/albums/ii116/personlyplus/th_31before.jpg (http://s262.photobucket.com/albums/ii116/personlyplus/?action=view&current=31before.flv)

before

http://i262.photobucket.com/albums/ii116/personlyplus/th_31after.jpg (http://s262.photobucket.com/albums/ii116/personlyplus/?action=view&current=31after.flv)
after

update car is still running fine.. and no return noises...

+1 on the seafoam.

your main problem was sludge on the outside of your lifters causing it to stick to the block instead of move freely.... a couple weeks of seafoam in the crankcase would probably clear that right up. and it is more sludgy at the rear of the engine (cyl #3 and 6) since its hotter there.

your misfire was probably made worse by the sticking lifters which cause a drop in compression which causes misfire. were you were getting (P0300?) random misfire..especially when you floor the accelerator -- the SES light would blink ?