Appiahtus

Hi guys I have the same problem on my Lexus RX400H. I hear a groaning vibrating noise when driving with light press on the peddle at about 20-30 mph on the engine esp up on a hill. This is not the case when on level roads and downhill or if I drive above 40-50 mph and on hybrid. My suspicion is the transmission. Don't know if changing the transmission fluid might help. Anyone dealt with an issue like that pls. Any ideas. Cheers