Diesel and Gas belt tensioners/idler bearings.

[Mekanic]

If you have some loud engine noise that sounds like a bearing noise, it just might just be bearing noise.
If you are changing your serpentine belt take a moment and spin those tensioners and idlers by hand to see.

I witnesses something a few months ago and forgot to mention it.

Nearly ALL tensioners and idler pulleys Take the same part number bearings.

GAS & DIESEL makes no difference. Part 6203 I go reminded of this a week ago when i noticed that my idler AND tensisioner Both had noisy bearings.
Most of the diesels (usually 8 rib belt) take 2 bearings in each pulley.
Most Gas models (usually 6 rib belt) have just One bearing

One of the school buses ate a serpentine belt a few months back it was a 94 Navstar DT466 and the idler pulley bearing failed.
Once it was fixed they looked the other buses and discovered many more that needed replaced Funny thing was
They ALL took the same bearings. (#6203) 17x40x12 mm
CAT 3126s
Cummins 5.9 & 6.7
INTERNATIONAL (Navstar) DT466 & DT466E
Even the Mercedes Diesel