Strange Static or Electrical Crackling Noise

A strange one this. Coming from the passenger footwell area there is a strange static electricity crackling type noise. It lasts about 10 seconds and repeats every 2 to 3 minutes. The stranger thing is it only happens at motorway speed with the wipers on. It’s not coming from the car audio system - that has been confirmed by myself and a mechanic. If you turn the wipers of and/or drop below 60mph it stops doing it. I’ve been in the passenger footwell looking for relays that could be causing it - but we’re still none the wiser. Just to add, there are no aftermarket add ons to this car that could be causing this either. I’m confused and my mechanic is. Has anyone got any ideas? Thanks.