First, I believe your trap is not to code on 2 levels:
1. The trap has to be within a certain distance to the drain.
2. There's a max allowable drop of the trap.
Besides not being to code, the way you have the trap now increases the risk of the trap siphoning - especially the vertical drop. Where's your vent break in all this?
I believe the sound yr hearing is the water dropping down to the trap and then being amplified by all that pipe up to the drain. If you're not noticing the trap siphoning (i.e., no sewer gasses when shower's off) then I'd just live with the sound.
If you ARE noticing gasses, then after yr done showering each time, leave one shower head on at a slow trickle for 1min. This will help refill the trap without causing such a rush that it siphons again.
And if you ever have to sell, do what you can to mask yr trap design to any inspector