The sound is likely coming from the airplane itself, as an added effect that might be supposed to happen on certain conditions.
It's possible the airplane code is detecting a condition that should only appear in flight which was instead a result of the airplane being frozen ( so it won't stutter because of two simulations fight against each other at every frame ) so it's generating that extra sound, which means the only possible way to fix this, is the developer using the published GSX variables to detect when GSX is pushing, and ignore whatever condition it's in place to call for that sound.
This might also possibly caused by another add-on, like FS Realistic, that might add its own sounds to airplanes, and if that's the case, it could only be fixed if FS Realistic checked if GSX is pushing, which has all the documented means to do so.
It also possibly related to the scenery, because I just tried a Pushback from our new KIAH at B12 with the CRJ, no strange noises at all.