50 Hz is very characteristic for a power line close by but as you already ruled ...

CompuHacker · on April 21, 2015

He's in the US, which rules out 50Hz at any precision. He needs to take measurements.

Someone · on April 22, 2015

On the other hand, without measurements, 60 Hz may be close enough to 50 Hz to include it in "around 50 Hz", depending on the OP's ability to estimate sound frequencies.

But yes, measurements are the way to go.

thyrsus · on April 23, 2015

On a piano, it's closest to the C three octaves below middle C, so around 33Hz. I regret my lack of perfect pitch.