When lights are on, the radio emits a low, but annoying hum. The noise does not get any worse when volume is turned up. Noise is not loud but sufficiently annoying to deter the use of the radio at night.
Any advice please?. Thanks.

Does the hum occur when the car is off (engine off, radio on), if it doesn't and fluctuates with RPM while the engine is running, its possible that you are receiving interference from your alternator. You can install an alternator noise filter on the power line between the battery and the alternator to minimize the problem.
I'm sure there are other problems that may cause the hum, and this is just one possibility.

When lights are on, the radio emits a low, but annoying hum. The noise does not get any worse when volume is turned up. Noise is not loud but sufficiently annoying to deter the use of the radio at night.
I had a hissing sound like this (ie only when the headlights were on) on FM channels in my AWD from when I got it, and it was rectified under warranty. I don't know exactly what was done to rectify the problem but the headunit was definitely replaced as a new radio security code was supplied.

When lights are on, the radio emits a low, but annoying hum. The noise does not get any worse when volume is turned up. Noise is not loud but sufficiently annoying to deter the use of the radio at night.
Any advice please?. Thanks.

does it increase at RPM?

when i had my factory unit in i had a 12v usb Charger that i used to charge my ipod
when it was plugged in, it made a whine relative to the rpm of the engine


there is a supressor avaliable from jaycar for this i think as has happend on other cars ive seen(non magnas)

Here's a great guide on what could be causing the hum.

http://www.crutchfield.com/S-QNXrAzeHhcF/learn/learningcenter/car/noise_suppressors_installation_guide.html

My head unit does so too, with the lights on. The engine on or off doesn't change the noise.

It's the TH, and it's an aftermarket head unit that came with the car.