[krischer]

50 Hz is very characteristic for a power line close by but as you already ruled that out I have no clue. Nonetheless much to high for any seismic noise.

[CompuHacker]

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

[Someone]

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