Try tightening down your carburetor. You may have moved it or loosened it when installing the K&N and air is getting in under it through a thin space casing a reed like whistling. Ask me how I know - I have a spacer under my carb and it wears out and whistles under acceleration until I replace it.
'95 ZJ Ltd. White 4.0L
TW DC DS; ARB Bull Bar, F&R Air Lockers; Superior Axles; Warn Winch; 4.56 gears; ACOS, Addco F&R Sw Bar, JKS Discos, Rr Track Bar & Nerfs, 249>242 Swap, Stillen rotors+Kolak pads, 3 pc. Armor, RE 4.5 Kit + 2.5"BB, Adj. UCAs&LCAs, OME SS, Bilstein 5150s, 15x8 AR Diamond Backs, 33 Truxus MTs, K&N FIPK, RE 62mm TB, Perf. Distr. 4.0L Kit, Exhaust: 2.5" Kolak exhaust FM 70: Optima Yellow, Mean Grn Alt., PIAA 520s, 540s, Kevins Rack, & TB Conversion