Soundproofing a Diesel

[79vette]

So I got a bottle of OptiLube XPD additive based on this study from the Diesel place forums. The conclusion was it was among the best lubricity additives on the market at the time (2003). OptiLube XL seems to have been introduced since then and claims to offer even better lubricity, so maybe I will try that one out next.
https://www.dieselplace.com/threads/...esults.177728/

I also added another layer of soundproofing foam over th transmission tunnel, and taped/glued the edges of my rubber floor mats together to make one huge mat. I also replaced the shift boot on my 4 speed with a much thicker one, and made a steel bezel to clamp it down to the tunnel (it was just installed with 8 sheet metal screws through the rubber, leaving a couple small gaps.

Doing the above seems to have made a significant improvement in the sound level in the cab. It's much louder than I like, but my cheap decibel meter on my phone now reads 67 db down from 78 db driving around at 35 mph. I don't trust the calibration of the meter app at all, but it definitely shows that the truck is much quieter now. I'll try to get it on the freeway next week and see if I can drive 70+ without earplugs...

My best guess is there was a lot more noise coming in from the shifter boot than I thought, and maybe the engine is actually running quieter with the lubricity additives as well. I've never been one to buy snake oils, but it does seem to help and the testing says it is good.

I'll still be insulating the hood, doors, and bedsides in the future so hopefully that results in even more improvement.