Mo ster 696 hissing noise

Pull the belt covers off and listen to see if the "whistle" is from the belts on the pulleys or a siezed/ sticking idler/ tensioner bearing.
Obviously check belt condition.
Do you have it's service history, ie: have the belts ever been changed or how old are they?
Your vid doesn't capture the whistle but belts or vacuum leaks are the likely culprits on any engine.

If it sounds like a slide whistle doing an impression of a goofy clown laugh, then your belts are tight. Change the belts, it's cheap ininsurance. I'd also recommend some aftermarket idler rollers. Super easy install and not too expensive.

my s2r800 makes a lot of 'air' noisea too. because i have opened up the air intake and exhaust and removed the heat shield. our simple 2 valve monster really dont sound the same as 3v or 4v liquid cooled bikes ducatis or other brands.

air spark fuel.
is your air fitler clogged, is bad fuel clogging up the injectors, check your spark plugs and look for markers
is it sluggish at all RPMs or just low rpm?