Finding out where the noise is coming from - XJ900

Since the temperatures here have risen into the 30s (90ish F), the bike has been running hotter (100°C or 200+ F on the gauge in the oil filler) and then develops a hollow knocking noise when idling. Although it runs smoothly and has good power, I'd like to find out if I need to replace or check anything, maybe even declare it as normal. So far, I'm fine with working on the bike, but when it comes to assessing engine noises, I'm pretty lost.
Context: Bought a sitting but revived bike with about 40k miles and since put another thousand on it.
What it could be searching around the forum:

• Cam chain loose
• Valve clearance too large (burned-up coal deposits?)
• wrong oil or too little oil

I put in 2.8l 20w50 with a new filter, and next want to check the spark plugs to see if it's running lean or something. Appreciate your advice.

Chris

 

If you use your phone and create a video you can upload it to a video channel like YouTube and post the link here so we can hear the knocking sound.

 

I can do that tomorrow. For today I replaced the plugs. Little bit confused about how the insulator tip looks. One half is white, the other half brownish. Is this because how the plugs sit in the head?

 

The look fine to me but I am no expert. They are tan and dry. I think that's a corona stain on the tan coloured insulator it usually happens at the bottom of the white insulator above the metal part the socket goes onto. Mabe slight mixture difference between the cylinders? Mabe one plug has done more miles than the other one. It is normal anyway I believe.

 

Wasn't that hot today, did the video anyway. It's not so audible, maybe at 0:43 it can be heard when I hold the phone near the pipes. Seems to be more permanent if the temperature continues to rise.
Maybe a broken exhaust gasket?
https://www.youtube.com/shorts/9Uq30KPuVXM

 

It's not the middle drive gear. Mabe a double skinned exhaust header or collector or silencer internal rattle.

 

Listening again the noise does seem to be at the exhaust area of the motor. Remember though it is air cooled. Are you used to water cooled bikes? I don't think their is anything wrong with your bike. Well not anything that is going to cause engine damage.

 

I'm actually only used to two-strokes. This is my first four-stroke. Definitely took me some time to get used to the valve noise. I guess with driving two-strokes you get hyper vigilant to engine noises.
I think you're right not to worry about it, but I'll check the exhaust next time.

 

My first two stroke was an MZ. Cheap but good transport. We're your two strokes liquid cooled?

 

Yes, liquid cooled. My first was a Sachs. It was sold around the late 90s, early 00s here in Germany with Yamaha engines.

 

I thought so. Yes the liquid cooled bikes run a bit quieter due to the water jackets around the cylinders.