Is it a whine or whistle?
If you push on the air intake and it changes or stops the noise, start there and make sure everything is seated properly and tight. The noise may exist all of the time, but you do not hear it at high idle (first minute or so after startup) or while accelerating due to engine noise.
Where and what did they spray? My guess would be that they sprayed belt dressing on the fan belt and are trying to charge you a lot of money to change a belt. There is no relationship between the air filter housing or intake tube and the camshaft. There is nothing you could spray on the outside of the engine that would do anything to the camshaft. Find a new mechanic.
'78 Scout SSII - 345, 4 speed, SOA, locked, 33's...
'06 LJ - 3" lift, 33x10.5R15's, Discos, TrueTracs, 4.56 gears...
'80 Scout II, '89 YJ, '79 Scout II, '93 YJ, '01 TJ, '00 Cherokee - all lifted, most locked and all wheeled