While riding the other day I noticed that when I engage the clutch my chain rattles in all gears. Do I just need to tighten the chain tensioner or is it something more serious?

You need to check chain adjustment asap before your next ride. Too loose a chain can jump off the sprocket and lock up the rear wheel (been there done that, luckily I was only going 15mph when it locked up.

Just to be clear, which chain are you talking about? The drive chain to the rear wheel or the chain between the crankshaft & camshafts?

Drive chain to the rear wheel. That is the only one disengaging the clutch would have any affect on right?

I had a rattle similar to what you described once... I cleaned, lubed and adjusted the chain really well and also cleaned out a bunch of crap from under the front sprocket cover. All was well after that, maybe that could work for you! 

I'm thinking the plastic or rubber protection on the rear fork (spring arm or whatever the correct term is), might be gone.

If your chain is stock, and hasn't been taken very well care of, you might want to swap it out. Mine probably never was taken care for, and was totally worn at 11k miles!

Same amount of miles I do on a cheapy chinese chain abusing it like crazy, yet with taking care of it.

Had something similar where upon engaging and disengaging the clutch there was a sort of rattle and clunk, turned out my chain had come to the end of its useful life, replaced front and rear sprockets and chain for peace of mind with an "ognibene silent sprocket" kit, feels great in comparison and no weird clunky rattly noises!

With regards the sprockets and chain kit; can't say much for longevity, only got a thousand miles on them, but all seems like good quality stuff and was reasonably priced as well, cheaper than a DID kit and has rubber bushings to reduce operational noise on both the front and back sprockets.

I had a chain rattle once,  most noticeable while coasting at very slow speeds.  It turned out to be runout play in the front sprocket (it was loose on the splines).  I put a homemade shim behind the front sprocket to make it tight against the snap ring, and the rattle went away.  It's been fine ever since.

 Hey ned, I have the same thing you experienced...what did you make the shim out of?

I made the shim out of a piece of flat clear plastic of the type used for the packaging of household batteries and toys -- the kind that's difficult to tear open because it's so tough.  Thickness is about 10-15 thousands of an inch (if I had to guess without measuring it).  I used a razor knife to cut the plastic into the shape of a large washer.  It's been behind my front sprocket for about five years and is working exactly as I intended. 

I changed the front sprocket last year, and the shim was not degraded, so I reused it.

The effect of the shim is two-fold:  it moves the sprocket outboard onto less-worn splines, and it removes all the extra space between the sprocket and the snap ring, so the sprocket has no latitude for run-out.

I use this same plastic material as a shim to keep my windsuring board's fin from wobbling it the fin box.