It depends where the sound is coming from.
If it's a synth just put an entry in BBE, stick the chord in piano roll for a complete bar/measure then drag it out as far as you like in Song Editor. I do it often cus I like drones.
If it's an audio file (.wav/.ogg etc) you'll need to choose a loop point in AFP carefully so the looping doesn't sound obvious. It's possible in AFP but I generally use a separate program Audacity for that sort of thing because it makes it easier to find clean loop points.
Steve