Top end noise under load

Hello everyone,
I've been able to get out on my bike a bit more recently (78 kz650 b2a) and I've noticed some noise coming from the top end somewhere. It almost sounds like a hissing noise but not quite, think hissing sewing machine? My valves are noisy at idle but I checked the lash using the factory service manual method and all were withing spec, with two being close to the lower tolerance on the intake side. I also adjusted my cam chain tension using the factory tensioner which didn't seem to help at all. I haven't noticed any adverse performance but the bike was like this when I got it so I'm not sure what "normal" power feels like. Anyone know what this could be caused by?

it could very well be your exhaust header leaking at the head. Next time it's cold, squirt some PB Blaster or similar penetrating oil on the exhaust fasteners. Then carefully make sure they are all tight. Only need about 10 to 14 Ft lbs of torque on them. Just good and snug not godzilla tight!
If that is the cause of your leak you should find one or more of them loose.

You can also feel around the header to head joints while it's at idle. You might feel puffs of air. Be careful! That area of the engine gets extremely hot after only a few seconds of running.

If exhaust it would pop on over run check around spark plug holes, some people hugely overtigten and stuff up the threads it will hiss from here if that has happened.