NDB beacon signals
From Signal Identification Wiki
You can find very similar sounds on the following frequencies:
-290 kHzKiloHertz (kHz) 10^3 Hz -293 kHzKiloHertz (kHz) 10^3 Hz -311 kHzKiloHertz (kHz) 10^3 Hz -317 kHzKiloHertz (kHz) 10^3 Hz -321 kHzKiloHertz (kHz) 10^3 Hz -323 kHzKiloHertz (kHz) 10^3 Hz -327 kHzKiloHertz (kHz) 10^3 Hz -342.5 kHzKiloHertz (kHz) 10^3 Hz -352.5 kHzKiloHertz (kHz) 10^3 Hz -355 kHzKiloHertz (kHz) 10^3 Hz -360.5 kHzKiloHertz (kHz) 10^3 Hz -400 kHzKiloHertz (kHz) 10^3 Hz -402 kHzKiloHertz (kHz) 10^3 Hz -404 kHzKiloHertz (kHz) 10^3 Hz -418 kHzKiloHertz (kHz) 10^3 Hz -421 kHzKiloHertz (kHz) 10^3 Hz
These are all found in the NDB utility band.