My vehicle makes this noise when you have the AC on or heater, defrosting system only. I only hear it when I have my foot on the brakes at a stop and sometimes when I am in park with engine running. I do not hear it when I drive. I have taken it to Cadillac I was told it was the water pump bearing, so I replaced the timing belt and pump assembly, I also replaced the Evap sensor per the engine light diag. Car still makes noise, so I took for a second opinion and I was told it could be one of the engine mounts or the transmission. they don't know, but they do hear the noise. the vehicle was always quiet up until this past year and u have spent over $1200 this year fix the noise. Cadillac told me it could be a hub, that does not make sense if the car don't make the noise when you are driving. I need to know anyone has this issue and could a Motor mount cause that noise. I feel no wants to help me find the noise since these cars are no longer made. I work for a car dealership, so you would think I would get better treatment.