Hello all!

I searched on this forum but could not find a match to the symptoms I am experiencing with our 2003 Disco so now I am asking for help.

There is a very clear tapping noise apparently coming from the front driveline. It's fast, I'd say as fast as the prop shaft, definitely a lot faster than the wheels. It appears as soon as I let go of the gas pedal, very clear tapping sound, and it stops suddenly when the speed is down to around 10 km/hr. It doesn't just fade off, it stops suddenly. It seems to come from the right side, under the floor, and from the front of the vehicle. Like under the passenger floor. It's not a grinding or rumbling or screeching or ringing sound, but more like a tapping sound, like maybe tapping on a large block of metal with a very hard piece of wood. The problem appeared over a few days after I replace the thermostat, following a cracked thermostat housing that caused a massive coolant loss. The engine doesn't seem to have suffered from that mishap, it runs just as good as it did before.

The transfer case was rebuilt four years ago. I replaced the oil and it was clean with no metal shavings, no bad odor or anything suggesting a problem.
I replaced the front differential oil, it was also clean and with a normal smell and some metallic paste on the magnetic plug.
The oil seal on the right side of the front axle was leaking a little, but not enough to cause the oil level to drop significantly.
The front propeller shaft was rebuilt six years ago and was greased regularly. I removed it, locked the CDL and drove it as a RWD, the noise is still there, if not stronger.
Pulling the parking brake or pushing on the brake pedal doesn't not cause the noise to go away of fade, and I replaced the front brake pads and disks very recently.
I cannot see anything loose in the engine bay, everything seems to be fine and properly secured.

So any idea?

Cheers!
George

 

Does it not do it when it's not moving?
Does it appear, stop, or change with engine rpm?

 

I recall learning a long time ago that knocking when the engine is slowing indicates a rod bearing problem, so sixpack's question is important- does it knock when you rev it and then take your foot off the gas with the truck not moving?

 

No, it only does it when I am coasting in gear, and it stops completely when I get down to about 10km/hr. It doesn't do it when idling at all, and it doesn't change with engine rpm. Only with the forward speed of the vehicle.

 

What about when turning?
CV joint maybe?

 

 

I removed the front driveshaft and drove around, noise still there.
The transfer case fluid was good, no metal shavings, no bad smell, still fairly clean actually. I replaced it anyway.

 

Turning while decelerating did not change the noise intensity or frequency. No perceptible vibration in the steering wheel when turning that would indicate a bad CV joint. Both joints are tight, no play, rubber boots are good.

 

I'll try that tonight.