Slaaneshi Havocs don't have to use noise weapons.
Just treat such a unit like a normal squad of marines (eventually equip them with a rhino) which got an option to have more special weapons.
As an alternative, the daemon prince could have the mark of slaanesh, and the havocs can be used with the mark of nurgle if you don't want them to use anything else than plasma weapons, meltas and flamers.
jwu