Random and intermittent " Bumping " sounds when recording with GT PRO

Help !
I have just installed my version of GT Pro on my new computer.
As a test....I made a recording of silence on track one to see what,if anything would come back through the speakers at playback
( I did,in the past,whilst at Gooseberry Studios in Soho ,run brand new reel to reel tape at top volume BEFORE recording on it, to check that the tape was not faulty.)
I found that an irregular series of " bumping" sounds were being recorded at odd intervals.
Is this because I haven't installed in correctly or,could this be other people,in the multi storey building I live in,turning appliances on and off ? Or what ?And ,can I do anything about it ?
Sam