The first step for getting rid of that humming cam chain is to install an APE manual cam chain tensioner. By installing this tensioner you will never need another. It's easy to install, and easy to adjust. Check it regularly.
Clean the bike so that no dirt or other things can get into the motor during the installation.

The tools you will need are:

Two 5/8 wrenches
One ratchet with 8mm and 10mm sockets.
One plastic zip tie
One razor blade, to remove any gasket that sticks to the block.
Move the carburetor vent hoses out of the way of the stock tensioner.

Remove the valve cover.

Get the piston on top dead center of the compression stroke. This is done to ensure that there is no load on the cam chain. You will know that the piston is on TDC of the compression stroke when none of the valves are being opened AND the timing mark on the cam sprocket is lined up with the arrow cast in the head (see picture at left.)

This way of finding TDC works for the CRF450. Other makes and models differ in details. See your manual for finding TDC for your engine.

Use rags to keep debris from entering the head when the valve cover is off.
It is important that the slack cam chain does not jump a tooth on the cam sprocket during the swap. Use the zip-tie (as shown) to snug the chain around the sprocket.

Now remove the stock cam chain tensioner and wipe away any dirt or oil. Using the razor blade, remove any gasket that is stuck to the block. Be careful not to cut into the aluminum with the razor.
Pull the new and improved tensioner out of its package, and show all of your friends. Back the 3/8 tensioner bolt out so that it does not push against anything in the motor during installation.

Install the new tensioner starting with the gasket. Torque the two mounting bolts to manual specs.

Begin screwing the tensioner bolt in gently until you can feel the resistance on the cam chain then stop.

Remove the zip tie.

Rotate the motor forwards, while screwing it in until you feel the cam chain drag against the cam chain guide. Then back it out 1/4 turn. DO NOT OVER TIGHTEN! (See picture).
Using the two 5/8 wrenches, tighten the jam nut snugly against the tensioner body locking it in place.
e Start the bike. bring it to an idle and listen for any clanking or slapping sounds coming from the cam chain. If you hear these sounds, leave the bike running and back the jam nut off making sure the tensioner bolt does not back out. If you let it back out you will damage your motor.

Tighten the tensioner slowly until the slapping or clanking goes away. BE SURE NOT TO OVER TIGHTEN!