I have a Fiesta Mk 5 bought new in 2002, 51 plate.

Recently I've been experiencing intermittent electrical problems and would like some advice on how to track them down, not sure they're related.
First is a "pinging" noise that sounds like it is coming from the passenger footwell when the lights are on. It's an electronic pinging or pulsing noise that is best described as being similar to the noise you hear from a strobe light or a camera flash recharging. It's intermittent and doesn't happen all the time, but I'm sure it's related to the lights as it stops when I turn them off and returns when I turn them on.

Second is a new issue that started last night, I've been getting very bad radio interference on FM frequencies. There has always been some interference on LW/MW but the interference on FM is new. I've eliminated the FM transmitter that I use to play music from my iPod.

Unlike the usual LW/MW interference you cannot hear any effect from revving the engine. It is also intermittent, can clear or get worse instantly, sometimes going over a bump can have an effect on it. At it's worst it completely wipes out FM stations with electronic crackle and interference.

Could this be related to dodgy wiring? A loose earth connection or a relay?

What is the best way to investigate?