Weird engine noise

[MattLong1232]

I have a '96 Civic 1.5. It's a new car so I can't comment on how long the noise has been there.

But under heavy load in the lower gears, such as going uphill especially or just harder acceleration, the engine makes a rattling noise, almost like a rattlesnake type rattle.

Thing is it only does it now and then, its not consistent. And i could be going along the road and boot it in 5th and it doesn't seem to do it. It's usually second and third gear.

Any ideas what the noise could point to?

I've read somewhere that it could be the Cat? also read it could be spark knock?

Any help appreciated...