I remember this vaguely - too bad MLB didn't investigate better at the time.

"The New York Yankees accused the Houston Astros of using whistles to convey stolen signs during the recent American League Championship Series, and it turns out they weren’t totally off base.

According to Andy Martino of SNY, MLB investigators did find evidence of whistling in footage of Game 1 of the ALCS, but they weren’t able to identify the source."

The whistle in that clip sounds clear, and it appears to come just before Yankees closer Aroldis Chapman releases a pitch. However, even with the whistles coming at vital points, it’s impossible to determine who was whistling and for what purpose.

In a subsequent video posted by Jomboy which features more pitches from the same at-bat, the original whistler inspired other people to whistle, which significantly muddies the water. Martino reported that after the Yankees complained, the whistling didn’t occur in Game 2."

At least the Nats were not victims of the cheating.