Volvo D13 Strange Idle

Hello,
On my 2013 D13 engine I have a question:
I have timed the below explanation 10 times while idling. The idle will change at 3 minutes and 14 seconds.

The truck idles normally and quietly. The idle will increase slightly and the engine sounds like it has a load on it, like having a PTO installed and engaging it (the truck did have a PTO but was removed before I purchased it). It may have been used for tankers.

1. From a regular idle, the turbo will spool up SLIGHTLY before the idle increases. The truck will then idle rough, the engine gets a little louder and will stay at that idle. I then tap the accelerator and the truck will return to its normal idle for another 3 minutes and 14 seconds and the process repeats as long as I tap the accelerator.
2. When I cold start the truck (truck sits overnight) it idles smoothy and quietly for about the first 45 seconds to a minute. After this amount of time the idle increases and gets rough like the description above. If i release the tractor brakes the idle normalizes and the engine is quiet.

Has anyone ever had the above problem with this engine? Any input would be greatly appreciated.

Thank you,
Teemobile

 

I'm not setting the high idle. I tap the accelerator to normalize the idle after it increases on its own. It's increasing on its own.

 

I don't think it's the Regen. I can be at a RR crossing caught at a train for 10 minutes with my foot on the brake and it does the same thing after a few minutes.

 

It does well with a load. I'm wanting to be pro active if there may be an issue. But if your saying it's normal then there's not much to be concerned with.
It does seem like it could have a bit more power when going up a hill with an overweight load.
If it is a warm up function wouldn't that rough idle disappear once the motor has warmed up?