Electric Motor switches to Gas motor almost right away and makes a loud sound

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It used to switch to the Gas motor at around 35mph now it does it right away when you step on the Gas or even after a few seconds while on idle. When it goes to Gas it makes a loud rumbling sound and the entire engine shakes a little. When it's on Gas it also rumbles a bit when you slightly step on the Gas. Anyone know what could be wrong? If I step on reverse while in Gas, no sound no matter how fast I try to go, only seems to be a problem going forward. No sound when a press on the Gas far enough and the engine runs smooth. It only happens when you apply a little gas does seem to matter how fast I'm going.