It's a Continental R-975 C4 radial engine. It sounds exactly like an airplane engine should. Shitty at idle and powerful at speed. Hearing it I could imagine an old biplane standing at idle waiting for takeoff then powering up into a massive roar. Not so easy on a clutch I bet. M4 was so tall because the engine is in the rear and the transmission is in the front. The drive line between the two has to pass underneath the turret bustle.
Radials always idle roughly. Watch the videos about WWII radial-powered fighters and you'll hear a similar sound. Since some of the cylinders are upside down (gravity working against fuel flow) and some are right-side up (gravity working in favor of fuel flow) a low RMPs there is always a tendency of rich mixtures in some cylinders and lean mixes in others. This evens out when the manifold pressure rises with higher RPMs.
The fact that the M4 could be powered by a variety of power plants was a great strength. Also in the desert in Africa when the M4 was introduced the radial engine was a godsend. No radiators consuming limited water rations and the air the engine fan sucked in provided ventilation for the crew stuck in a metal box under the sun.
The proper designation for this tank should be M4A1 76(W) - M4 medium tank w/cast hull and R-975 engine t-23 style turret with 76mm gun wet stowage.