strange top end noise has me baffled.

I was hoping someone could help me. I have just got my bike together after a new top end and now it's making a strange 'tinging' noise.
The noise seems to be coming from behind the left side kips cover.
the pitch of the noise can be reproduced by flicking the cover with my fingernail.
the noise is most noticeable at idle and can't really be heard while riding.
When I put pressure on the cover with my thumb, the noise goes away.
The cover is not loose, but seemed to get very hot quite quickly, and when I removed it I noticed that the kips valve was positioned so that exhaust gas was directed straight onto the cover.
I initially thought it was detonation but not any more. The bike is stock, the jetting is stock, I checked the timing with a light, and the line on the flywheel lines up with the mark on the cases.

This is my first top end on the kdx and I really tried to do everything right, I read all instructions, cleaned the power valves, used new gaskets, torqued all bolts, and had no parts left over. Any help anyone can give would be very much appreciated.

Thanks.

It,s a 1999 KDX 220. When Installing the exhaust I used the o-rings and high temp silicone (as I've done on all my bikes with good results) to seal it. I've looked closely and haven't noticed any leaks. The bike did not make the noise before I replaced the top end so I think it's something I did.

Thanks again.

See, my concern is that the KIPS is not installed correctly, diverting too much exhaust gas into that chamber. Is the KIPS cover supposed to get too hot to touch after 30 seconds of idling?

Funny you should mention that pipe gasket, it wasn't there when I took the bike apart to do the top end. One came with the gasket kit I bought and it looks kind of like an FRP torque ring. After some thought I left it out because it ran ok without it and I was concerned that the change in header length would alter the power characteristics. Holding the pipe with gloves doesn't stop the tinging noise, but maybe I'll install the gasket anyway so that I know it's done right.

Yes, it is the athena kit. I am now thinking the noise could be a side effect of the engine not running clean. The jetting seems to be rich, it runs flat on top, it smokes too much, and the exhaust note seems a little raspy and poppy. Could there be a small head gasket leak? the coolant level hasn't gone down and it starts 1st kick. I didn't use any sealant, just a new gasket. To add insult to injury the piston I took out was fine, (it was marked SK, ever hear of them?) I wish I had left well enough alone.