As it has been said before, the cam chain tensioner on the 350/400's isn't too swell. If you look in the picture that was posted before, that spring is what is supposed to apply tension to the chain. When you loosen the lock nut you allow the bar inside the spring to move freely and the spring should take up the slack in the chain. Two things can happen over time: 1) the spring loses its "springiness" and no longer has the necessary force to tension the chain ans 2) If the motor is run with a lose camchain for too long it will rub against the joint in the horseshoe and eventually freeze it in place (as stated above).
What you have to do is essentially "help" the spring out a bit.
1) remove the bolt on the topside of the oil filter cover (the one that covers the top of the push bar)
2) Loosen the lock bolt and start the engine
3) Using a small screwdiver or something similar, push down on the push bar. The screwdriver has to be fairly narrow because the hole that the push bar actually moves in is quite small. I sometimes use a small precision type screwdriver like you would use on a pair of glasses or small electronics. Also, you may have to give the screwdriver a pretty good heave like I did. Your horseshoe is probably a bit tight and needs a good push to break it lose.
4) Push down just until the noise goes away. It is possible to apply too much force on the chain which could cause it to snap.
5) While you are pushing down, have a friend tighten the lock bolt. (I haven't figured out how to do it by myself yet)
6) Tighten the lock nut, reinstall the bolt from step #1 and ride.
Note: I always wear thick leather gloves or potholders to keep from burning my hands on the header pipes while I push on the push bar.
I think you will be amazed like I was at how quiet the motor becomes once you do this.
