valve train chatter

Hi all,
I recently purchased a 1994 XJ 4.0 automatic 4WD 4-door with 130,000 miles. It had been stored outdoors for two years without being started. I changed all the fluids, (Mobil 10w30 for the crankcase), installed new plugs/wires/rotor and cap, and it started up on the first try.
It runs beautifully in town and on the highway - except for one thing. I hear what I believe to be valve-train chatter when I accelerate moderate to hard.
The chattering increases as I accelerate and decreases as I let off the gas. In other words, it chatters "up" and chatters "down."
The oil pressure is 50# at startup and 20-30# at warm idle/highway.
Maybe it isn't a valve-train problem at all...
Any ideas?
Thanks,
Dale

Try a higher octane fuel (93). Fixed me right up.

Could be one or more of the hydraulic lifters are "stuck" and you now have excessive valve clearance. It might eventually go away as the sludge in them breaks down, now that you are using it. You could try an oil additive or some Marvel Mystery Oil.

may also want to check the knock sensor, it retards the timing when detecting a knock. Watch the scale with a timing light while tapping on the block with something beside the knock sensor, the timing should retard, if not its bad.

All,
Thanks for the great ideas.
It had occurred to me that it might be timing knock (marbles in a can), but it has been years since I have heard timing knock and I thought that it would only make noise when you accelerate and stop when you let off the gas. I will try a higher grade of gas tomorrow and see what happens.
As far as a 'stuck' lifter, I drained the 10W30 oil today and refilled with one quart of Rislone oil treatment and five quarts Pennzoil 10W40 High Mileage Vehicle oil. I just drove 30 miles up and down I-91 at 65 mph and the oil pressure stayed at 30 lbs. at 200 degrees coolant temp. We'll see what happens with that, too. I'll keep you posted.
Thanks again,
Dale