FL's sampler/channel settings has a Resonance knob.....but by default, it is off........best way to do what Xabiton said is to send one copy to 1 mixer track....and send the other copy to the next mixer track......and load up an instance of Fruity Parametric EQ 2....and if you don't know how it works, open the FL Studio Help/Manual...it explains it pretty well..cuz the Parametric EQ 2 has like 7 or 8 different types of filters to choose from...so its good to know what each does, and when each should be used....but in your case, a low pass should do the trick as i stated above.