I feel like the sound for the T-Series is too quiet, it just sounds like a normal car engine. It should sound more like this.
The entire sound system is currently WIP. The T-series in particular is planned for an overhaul. For reference though, here is a short video of a 1999 International Eagle starting up and idling:
I can agree its off, of course, being from Ohio, I hear and see a lot of semi trucks, the T-series, sounds nothing like certain Peter-Built/ Mack Trucks, maybe it's because I'm from America but I've never heard this tone of semi engine, not enough torque in my opinion.