On my 1999 Camry when I'm going up a hill just enough that there is strain on the motor and it kicks back a gear to keep up to speed , I will get a strange vibration noise as if the strain is vibrating my muffler ( which could be the case???) So as I'm still excelerating up the hill , I slack off the gas a bit and then push back on the gas again, it will be gone. This will only happen whenever I'm climbing a hill! On a level road I can throttle the car and never any strange noise! Any ideas or suggestions all wise ones!