My 1999 xj8 runs great but it makes a clattering noise when I push on the gas. I have changed the oil, and checked the water levels. What could this be?

sounds like nocking and pinging = cheap gas. Try running 93 octane or a different brand if you are already running 93. Sometimes caused by carbon build up in the cylinders.

Check engine oil consumption; if it's heavy, this is a dead giveaway for what's going on. Get shot of the car pronto! One word--Nickasyl.
Leedsman.

I tried the better gas and it helps a little sometime. If the engine oil consumption is heavy, what does that mean?

If you can adjust the timing, retard maybe a couple of degrees this will also help with the pre ignition (clattering when you step on it) Mark the distributor before you move so you know where it was, check the timing to spec of car. Hope that will help you out.

No distributor. Perhaps you could rotate the Engine Control Module some.

Seriously, no codes? If indeed it is pre ignition the knock sensors could be failing to cause the ECM to retard the spark advance but that should set a code and illuminate the CEL.

Oil consumption has several potential causes, all semi permanent and expensive. If you are using oil change to a higher weight oil. Try 10W40 or 20W50.

Nickasil plated bores in Jaguar V8 engines up to about model year 2000.
This is a plating direct onto aluminium or alloy bores, around 5 to 10 thou'. It's extremely hard, effective, and used by a number of makers, notably Porche.
UNFORTUNATELY, if high-sulphur petrols are used, the plating is gradually eroded until the engine's oil consumption becomes a problem. This can happen even at 25,000mile.
Jaguar has had to replace hundreds of engines due to this effect. There is no other way to fix it.
Symptoms are a very heavy oil consumption, and another giveaway is "blow-by". When engine is hot and running at full temperature, lift the oil filler cap just a little carefully. A big 'wind' coming out indicates the blow-by, i.e. air and conbustion gasses are blowing past the piston/cylinder wall and into the crankcase, then venting to atmosphere when you lift the filler cap a little.
As I said, if this is happening to your car, either replace the engine with a good second-hand one, or get shot of it!
Leedsman.

Thanks for all the help. The car isn't using any oil so I will try to adjust the timing and see if it helps. I love this car and no engine lights are on. It only has 66,000 miles on it. Thanks

Hmmm, no oil consumption, but clatters, tensioners perhaps?