I have a 2000 nissan xterra with 92000 miles. I get the oil changed every 3000 miles since I've bought the car in 2000. I've noticed that when I travel under 40 miles per hour I hear no noise. But when I travel over that milage I hear the tapping noise. It will go away if I travel around 25 miles per hour for a mile to two. What can I do to eliminate that noise?

 

probably need to have the valve lash adjusted

 

The problem can be numerous things. Daniels post is a common problem but if it were your valves that need to be adjusted it would more then likely be heard all the time then jus at the 40 mph mark. More info on the problem would help tremendously. So it doesn't do it at idle speed or during its first start up of the day? Could possibly be a wore bearing or something. Are you sure it's comin from your motor and not from your suspension?

 

Sounds like the hydrauylic chamber cannot refill with oil fast enough. That it can at slow speeds, and have enough oil in one, or several, to keep pressure built up while fighting the force of the valvespring acting against them. But with increased speed, the increased speed of the valve force is too much for the little oil re-entering the adjusters. That is my guess. Just a guess.

I'd try cleaning additives in your crankcase for a while. Then maybe switch to one of those 'motor honey' products sold by various names like Motor Honey, Lucas, STP, etc., that raise the viscosity and are called oil stabilizers. (You can add 20% of your oil volume, these mfg.ers all say.) Then again, maybe thinner oil will work better for you.

If all else fails, try adjusting later, if you even can, but not overdoing something to where some valve hangs open and causes low compression/misfire/backfire/possible burned valve.