The number is around 60, give or take, with a rumble that is part of the soundtrack to an already unique US Open.
About half the time, it comes from the north behind the 15th green and near the 16th tee where the one tree at Chambers Bay stands guard.
The other half, from the south near the 18th tee. And in between, it's unavoidable for those playing either the 16th or 17th holes. Of all the variables that go into conducting a US Open, this is one the US Golf Association could not control.
Along the shores of Puget Sound between the edge of the Chambers Bay property and the blue water, stands a rail corridor vital to moving freight and passengers north and south through the Pacific Northwest.
That rumble of clacking rails was just as much the sound during this US Open so far as the roars from the crowds.
The operators of the train lines, BNSF, and the USGA had discussions on whether schedules should be altered for the four days of the tournament. The final decision: Nope, continue with the scheduled service.
On a golf course already unique for its design, development and features, the trains are one more quirky element. The tracks are owned by BNSF, and run from Vancouver, British Columbia, all the way to Eugene, Oregon. All told, roughly 60 trains pass the course daily, with half going by between 8am and 8pm, and the other half working the overnight shift.
The only element missing is the signature whistles. When the public is playing Chambers Bay, engineers regularly sound on the horn. BNSF instructed their engineers to keep the whistles silent unless it's an emergency.