I have a friend in Bulgaria who has had to have an aftermarket relay fitted to her car for her head lights as she couldn't get one from a dealer but now when she swatches her headlights on she is getting buzzing a noise from the relay, does anyone know why this might be? perhaps Tony could answer this one but anyone who can help with an answer will be very much appreciated.

I can only offer a guess on this I'm afraid. I think it might be something called lamination hum, it quite often occurs on larger relays or coils. It can be noticed quite a lot when you go near an Electric sub station. A buzz can also be noticed if something is not bolted down properly and the rattle that is produced sounds like a hum.

But like anything else if in doubt get it checked out, but personaly I have never come across hums from low voltage gear, well not in a car.

Tony

The usual reason for a chattering relay is that the voltage applied to its coil is marginal to hold it in. Two causes. Either the relay coil operating voltage is too high (eg in this case, say a 14V relay instead of a 12V relay) or the applied voltage is marginal (eg in this case the battery is low - but that would also affect other relays - or there is a voltage drop in the circuit supplying it).

The effect is like a see-saw. The coil pulls in and due to the characteristics of inductances, when it's pulled in the current drain is too high for the supply circuit so it drops out again...the circuit recovers and the relay pulls in again....and so on.

There may be other subleties caused by the way the relay is wired in but without knowing the modified circuit it's difficult to speculate.

The manner in which the relay contacts are used won't cause chatter.

Nice explanation nets