Toyota Celica 190 Engine Rattle, Need Help Badly!

[Short190]

I have a 2002 Celica 190 with 116,000 miles on the clock, as far as I know the chain hasn't been changed.

I used to have a rattle that would only happen at 2500rpm and 3500rpm regardless of what gear I was in. Recently I pulled away from a set of lights, changed into second and put my foot down, and just as lift engaged there was a noise from the engine, caused me to back off the throttle, and now the engine sounds like a tractor. I'm sure something is rattling in the engine, I have had the rocker cover off today and inspected the camshafts and cams and they all seem fine, no snapped bolts, and I also inspected the chain, and there is only a slight bit of slack in the chain, but there is some slack as you try to move the chain from left to right, don't know if this is supposed to be there?

If anyone else has had the same problem or similar problem any help would be much appreciated! I don't want to have to get a new engine for it or get rid of my Celica.