So every now and then we get an issue where the Sub in the home theatre hums, and the LED lights in the lounge (seems to be lounge only) start flickering.

It seems to be at the same time, although while it was happening tonight I turned the sub off but the lights continued to flicker, so I'm not sure it's sub related (although I wouldn't count it out).

Last night when it happened I noticed a train was going past at the tracks down the hill (Wellington electric trains).

Any ideas what it could be? I haven't asked the neighbours if they've experienced anything but because it seems to be only one area in my house i'm assuming it's my issue.