4 cylinder engines fire every 180 degrees, 3 cylinder engines fire every 240 degrees, 6 cylinder engines every 120 degrees. Many small explosions versus a few large ones, for the same power, are inherently smoother.
The cabs on a B3350, and an LX 3310 are much louder than the cab on a L5030. All tractors are HST. The majority of the noise in the B3350, and the LX3310 are HST whine. The cab’s on the L5030 and the M100X are quite and comfortable. The smaller cabs on the compact tractors are more resonate to higher frequencies because of their size. I use noise canceling headphones when operating smaller tractors.
Noise canceling foams, available at aircraft supply sites, help reduce the noise. However, the dimensions of the cabs still determine the resonate frequencies.