The M155 are known to be noisier than the M150. On the late M155 (2015 & 2016 models) they changed the cab mounting and added rubber isolation pads under the hydraulic valves which is supposed to help. There was a "Cab isolation kit" available to upgrade early M155 that were still under warranty if a sound test was done and sent to the factory, and the factory determined that the offending noise was of a frequency that the "kit" would fix. The "kit" is very expensive (over $2000 without the day of labour to install it), and includes different cab shocks, different shock mounts with additional rubber cushions, and isolation pads for under the hydraulic valves.
I finally had a chance to test our M155 for noise levels (using a stand-a-lone Extech 407738 sound level meter) after the "Cab Isolation Kit" was installed and compared it to the "Before Installation" levels.
BEFORE: 1800rpm was 69db 2000rpm was 66db 2300rpm was 69db
AFTER : 1800rpm was 61db 2000rpm was 61db 2300rpm was 64db
As noted a 5-8db reduction in noise is VERY noticeable. For me in our machine, even though the "before" readings at 1800rpm and 2300rpm were the same, the TYPE of noise at 1800 rpm was more annoying to me, and I never ran the machine at that speed because of it. Before testing, I used ISC (Intermediate Speed Control: which is Macdons' version of "Cruise Control") at the 2000 rpm speed, and the initial test results showed why I preferred that speed (it was quieter).
It should also be noted that even before the "Isolation Kit" was installed, the unit was not 'REALLY that loud. When you look at the Nebraska tractor tests, most of the later articulated 4wd tractors are in the mid-70s', with some of the front wheel assist tractors making it into the 60s. As I stated earlier though, sometimes the "sound levels" don't tell the whole story if the "type of sound" is not also taken into account.
I have never tested a M150, so I do not know what their readings would be, but I would call our M155 after the kit was installed to be VERY QUIET.