The loud clacking sound that comes from the chain hitting the rub block (normally heard on deceleration or slow speeds) is more than just irritating, it may be wearing away the aluminum from the top of the swing arm. This is a serious problem on all newer YZ's/WR's (97'-02' 125, 250, 250f, 400, 426)
The swing arm rub block retaining bolts that go through little inverted top hats are suppose to locate and secure the rub block. However, the design allows the top hat to seat against the swing arm BEFORE a significant clamp load is applied to the rub block.
This, along with bolt spacing that is rather wide, allows the rub block to slide laterally as well as bounce on the swing arm each time the chain smacks it. The rub block is actually wearing away the swing arm material at the contact surface. To inspect your bike, remove the two bolts on the upper side and lift the rub block. The contact area on the swing arm should be perfectly smooth. Mine was pitted and worn down slightly (with large grooves).
1) Modify the top hat shape to provide higher clamp loads. The brim of the inverted top hat needs to contact the rub block before the top hat bottoms against the swing arm. Rolling the brim towards the top of the hat, which is toward the rub block as installed, will ensure that it contacts the rub block and maintains clamp load as the bolts are tightened. Place the top hat brim against a 9/16-inch socket with the hat portion sticking out of the socket. Then, using light pressure from a vise (or hammer?), gently roll the brim towards the top of the hat. How much? Enough to let the brim dig into the rub block and hold as you seat the hat against the swing arm. I'd guess about 2mm. 2) Provide an isolation material between the rub block and the swing arm to cushion the impacts and help secure the rub block. I used a bead of silicone on top of the grooves and pits. Then I found a piece of foam (about 1/2"thick) and placed it inbetween the swingarm and the rub block (on top of the silicone bead) this helps keep in in place and keeps the rub block from rubbing a hole in your swingarm!
To do the job I had to remove the lower chain roller and c/s sprocket, and then roll the chain off the rear sprocket. A long hex key is required for the lower front bolt - I had a long 5/32-inch that fit the 4mm just fine.
This modification will eliminate the chain slap at low speeds and will keep your swingarm from cracking or getting a hole in it! Like this: (notice the "hair line" crack on the weld of the swingarm, this was caused by the chain slider after 2yrs of riding...very dangerous)