The ideal situation for recording a sound is in an environment completely free of any extraneous unwanted sound or noise. 

One obvious source of potential noise is the equipment being used to do the recording with. Tape machines make a certain amount of noise as they stop, start, and run. Even electronic equipment with no moving parts can create audible hums which could find its way onto your recording. Then there's ambient noise from the environment, people, traffic, etc. 

The solution to this problem is to provide some form of isolation from these extraneous sound sources during the recording process. Professional recording studios provide a separate acoustically baffled room to record sound in, with the equipment and technical personnel in a separate control room. 
On a more modest scale, if recording must be done in the same room as the recording equipment a few steps can be taken to improve the chances of making a cleaner recording. 
Make sure that all doors and windows are closed to help keep the outside world and all its sound out ofthe recording environment.  If possible use close microphone technique as this will increase the ratio of wanted sound to any unwanted extra sound in the environment.  The use of some kind of baffle between the recording equipment and the microphone can help reduce the amount of equipment noise going to tape.