Knocking noise under dash Volvo 780 2017

No not at all. The engine just wanted to be hotter at idle so It bumped up the rpm to warm up. It's only to meet the "clean idle" certification. Some engines use urea at idle to keep emissions down, volvo controls engine temp.

It's no different then if you raised the rpm yourself.

 

I can't compare numbers with you. My truck is a heavy spec XE 16 hauling 144,000lbs so my fuel mileage is only about 4.2mpg

 

Just to revive this post for anyone who may be having this same issue;
I also had the knocking behind the dash. So I called a Volvo shop and asked about the actuators. I was told that its best to buy the entire housing because you have to take the dash apart on the passenger side to get to the issue and don't want to find its another problem and not the actuator.
So the blend door housing was 109 at the dealer..not too bad. This is what it looks like. The bottom where it says Volvo houses the blower. The top is the housing that has the 3 blend doors., where the orange plug is attached to. (That's the new one installed).

The pic where I am holding the old one, you can see the two actuators. If these are bad, Volvo sells them for 20.31, (I was quoted) also reasonable. But there is no way to know if they are bad until you rip everything out. So they are cheap but I didn't want to run the chance.

On the close up image You can see what was causing my knocking. The gears came undone for some reason one one of the blend doors and was trying to "catch".

I can actually fix this and could save saved 109 bucks. But again how could I know.
So the job isn't too hard but When I put everything back together, I forgot 1 stinking piece of the ducting, so I had to take everything all apart again..lol I did it all under 3 hours. Not bad for my 1st time.

BTW, I didn't take a pic but the ducting sits on top of the 3 blend doors.

Hope this helps someone solve the knocking mystery.