Yes, it is probably EMI either entering directly into your system or EMI affecting your power lines. In any even, it is better to turn everything off during a storm. Even better is to unplug the gear because a direct hit of lightning could be powerful enough to jump the switch in your gear.
lightning storm
listening to my new Maria Callas vinyl last night and a huge thunderstorm/lightning storm starts up. About 2,3 seconds before each boom, I was getting a lot of static through my speakers. EMI from the lightning? I have no idea so asking the group. Thanks. Smart ass answers accepted also :-)