manufacturing tolerance can cause some whine. In other words, we are talking about a few hundred thousandths of an inch tolerance--closer tolerance than most machinery is able to achieve at a mass production level. So one that is .00006" larger than the other, that one might whine a little.
This is why super udt is imperative. Many don't understand how the hst works, and how close the tolerance between the pistons and cylinders are. Remember, oil doesn't just lubricate it also cleans, protects, fills in the gap between moving parts, cools, and it has to flow at the temperatures that the manufacturer wants it to.
lastly the most complaints I ever had with hst noise had to do with two main causes. First one was addressed above and secondly, using high gear for everything when there are two other gears that will reduce the noise and more importantly the heat produced. If you are running a shredder at 5 mph, you can use low gear and have more control over the tractor, reduced heat, and a lot less noise.
With all that said, the L3301/3901/4701 are all considered "standard L series" tractors, which are differentiated from the GRAND L series tractors by a lot of things, one of which is a huge reduction in noise, vibration, and harshness. The GRAND L series costs a lot more, but you get a lot more.