Do you mean static, or noise, or interference?
Static is a scratchy intermittent sound - like the sound of lightning picked up by a radio, or metal to metal contact somewhere.
Noise is constant and sounds like a waterfall.
Interference is a distorted, 'crunchy' sound.
Is the sound very regular i.e. always every so many seconds, or irregular?
If it is static, it could be a lose earth somewhere on the airframe. If it is noise, that could be interference from something getting into the intercom or the radios.
Could it be the C152's charging system? A strobe light?
Does your airfield have a large radar dish? This could produce interference as it sweeps round.
Edit: Hi Jan, I was writing my reply as you posted yours
