i have no direct answer to your question, but for those with no access to the numbers, you're talking about the Marx (3991) diecast tender. the top/ sides is a one-piece metal casting with no openings which sits on a simple flat tinplate frame (6.125" x 1.75"). the internal headspace is probably about 1.25 - 1.5" closer to the front. the two 4wh trucks are rivet mounted with a free space between trucks of about 2". no holes in the frame so something would have to be opened up or you'd have a pretty quiet whistle.
no electrical pickups on the trucks.
my indirect answer... put a whistle in a boxcar/ express reefer. right now i'm watching a movie on a computer screen with sound coming from speakers off to the left, but my mind wants to believe the actors are speaking. ...go figure.
good luck...gary