I experienced this same problem.
My primary home's master bath has a shower head, rain head and four body sprays. Apparently based on the anticipated water volume the plumber installed a 3" drain pipe with a 30" (approx) drop to the trap.
You could hear the shower drain on the other end of the house! It bothered the heck out of me for nearly three years.
Like others, a little experimentation determined that it was distance the water fell before making contact with the standing water in the trap. That 3"x30" pipe was a perfect echo tube.
Based on the same principle of a Zen Fountain, I figured if I could get the water down in "steps," rather than in one long drop, the problem would be resolved. It was.
My initial though was to find a massive drill bit. The water would wind down the spiral and make no noise. If a drill bit could be found it would be extremely expensive.
My ultimate solution was to build what I now call the "drain muffler."
Since a picture's worth a thousand words, here's a picture:
Since your drain is 2" rather than my 3" drain, you'd have to use a different diameter "Support pipe" than I did in my prototype.
You'll notice large "notches" cut into the bottom of the muffler. These are to allow water passage to and through the trap.
I'm just an inventor -- not a plumber. Please forgive me if I have used incorrect terminology in describing my solution.
I'm confident that buildign a "drain muffler" will solve your problem.
Delray Beach, Florida
Note: If I ever make a version v1.1, the 1/2" PVC Steps will be every 45-degrees creating a helix rather than a criss-cross pattern. The helix will ensure that every drop of water is diffused by a step before hitting the trap.