It it's a digital system then the signal strength doesn't matter for the background noise level. Start a recording and play the file on your computer and compare. It it sounds fine then the audio circuit in the scanner are adding that noise and are not normal.
The speaker are a very small one only capable of reproducing high frequencies, where the noise are, using an external speaker could help. If you are using earphones then if those are too sensitive it will make background noise from the scanners electronics more evident. I use an extension cord that has a volume control on it so that I can reduce the level so that no noise are heard and then adjust the scanners volume to a suitable level.
/Ubbe
