Rattle/Loose Chain type noise

Hey everyone. I've had this noise since I bought the bike, ~6000 miles ago, I think it's coming from the font cylinder although I can't be certain.

It sounds vaguely like this "rattlesnake" in this video at the 0:20 - 0:25. Yes, I know it's a video of a rattlesnake but it's the closest sound I could find as recording it is virtually impossible for me.

I only hear this sound below 5 - 6k RPMs in any gear except 1st. And it only happens when accelerating, the sound completely goes when I come of the throttle. Like I said, it has been there since I bought the bike and doesn't appear to have gotten louder or changed in the ~6000 miles I had the bike.

It could be a normal sound the SV makes or a sound that shouldn't be there. So some input would be appreciated. I'm going to take both CCTs out over the winter months and have a good look at them to make sure they're in good working order.

I have a slight leak from the rear CCT adjuster cap thing (sorry, don't know correct terminology) probably because I haven't done it tight enough, but whilst tightening it the first time it felt as though it started to thread, if that's possible with the adjuster cap, so I backed it out slightly.

The tapping noise I had before is still there, no difference in volume or anything. However after more inspection it almost sounds as though it's louder near where the downpipes/headers meet up, could it be the metal perforated pipe I saw in there when swapping exhaust making the noise? Maybe it's come loose or something, I don't know. Will have to take the can off and have a look.

Anyway if someone could clear up what the "rattlesnake"/loose chain type noise is it would be much appreciated.

Sorry for the wall of text, just want to get everything out there in case it helps with diagnoses.

I have a quick question regarding my leaky rear CCT cap. I took the cap off earlier and a thin plastic washer fell off with it, I didn't have another plastic washer so I replaced it with a copper crush washer of a similar size. Is it OK to use a copper crush washer here or is there a reason a plastic washer was used?

Hopefully this has sorted out the leak as the oil was somehow getting from the CCT and then covering my chain and left side of swing arm in oil making a right mess.

Any heads up on the noise described above would be good. Thanks.