Transmission noise in 8 high

I'm hearing a subtle grinding noise from what I believe is the transmission when I'm in 8 high and going through a dip in the road or when I brake going downhill. It only seems to happen when my foot is not on the accelerator. I've tried going the same speed in 8 low over dips and don't hear anything so it's just 8 high I think. I thought it was my ride height cus I had a bad leveling valve last week and when it was too low I heard an awful noise in 8 high when not on the accelerator. I quickly rigged it up to proper ride height and didn't drive more than a mile after hearing that noise. I now have a new leveling valve and I had Peterbilt measure and confirm my ride height is set properly so I'm really confused about this. Maybe I'm over thinking it but ill attach a video of the sound. You'll have to turn the volume up all the way and pay attention to when Im off the accelerator and hitting the dip in the road

 

Not yet I only noticed this starting this week but I will get it checked for sure. Last time it was checked was 3 months ago

 

I was wondering if it was just a speed thing but I've ridden in 8 low at 60mph over bumps that would normally make the noise doing that in 8 high and haven't heard anything. I don't hear anything in 7 high either. The temperature is also normal on the gauge. There's also an accompanying dull rattling noise from the shift stick that I hear in all gears that I'm unsure if Ive only just noticed because I'm concerned now or if it's a new sound. It's quiet and will be drowned out if the fan is on but my brain is really honed in on it and I can't unheard it now haha

 

Transmission noise

I was able to upload a video of the sound when going over a bump. I will only hear this if my foot is off of the accelerator and I'm coasting. When going down hill I hear it slightly grinding too and noticeably so when braking down hill. But again only in 8 high while coasting

 

I think I resolved the issue I went and measured my ride height and it was an inch lower than what was specified for my truck. My trailer is now sitting at 13' 7.5" tho isn't that too high for standard dimensions?