It's not a GSX problem, it's a difference how P3D handles positional sound for AI vehicles.
Those sounds you are hearing are the default engine sounds vehicles, and are handled by the sim itself, they are just louder in P3D than in FSX, but there's nothing in the GSX code that calls for them, it's all handled by the sim.
I GUESS, you are hearing them more from GSX vehicles, because they are much closer to your airplane so, perhaps, this is a result of a different distance attenuation calculation between FSX and P3D, not sure if we can really fix this, but it has been reported to LM, even not related to GSX.
