I'm working with some audio and some of it has background noise only during the dialogue. I remember this having to do with the way the mic was being used. Something to do with the limiter? I just can't remember the specifics.
It's either a non-optimally set up gate or limiter acting like a gate. Basically, the noise is present in the room constantly, but whoever recorded it wanted to reduce this. With a gate, any recorded level over a set threshold value opens the gate - set the gate threshold for just above the noise floor and you eliminate the noise unless someone speaks or makes a sound above the noise floor. The problem with this is that if there's a lot of noise, the threshold has to be set high and you get the effect you're talking about. I much prefer decisions like this not be made on set and instead let the audio post deal with it. You can do a lot on set to minimize noise, but applying gates to your mics as they are recorded isn't optimal, since you put the next person (in this situation, you) in a spot. The only way to recover (assuming it sounds unusable) is ADR.
In addition to Mark's excellent points, the noise could be from a really noisy hardwired mic, from a noisy wireless rig, from a mixer with the phantom power switch inadvertently set to 12 VDC for a mic that requires 48 VDC, or from bad gain staging, from one of those "adapters" made by Juiced Link or Beachtek, or from an audio chain with an input set to supply phantom power to a source that doesn't require it and isn't happy about it,
Before you bang the person responsible, find out what problems he/she may be facing.