Oh Oh....something in my coolant reservoir

2014 Volvo D13 448,000 miles just noticed something black in the bottom of my coolant reservoir tank. The engine is cool took the cap off and had a small bit of pressure....
This came out of nowhere - before this was noted changed my fuel filters...the water separator looked a little black but I thought it was algae that I have been treating. The spin on looked okay. Funny also just sent in a oil sample for analysis yesterday ..routine not because of this.... and now today look what I have. No miles on the rig since I changed the filters...
Yesterday was testing my fan clutch (getting it replaced tomorrow - so will dump the reservoir and figure out if it's fuel or oil....) and did note some bubbling in the reservoir coming through the small line that goes into the side of the reservoir tank and looks like it comes from a block near the EGR coolant area?

Before I bought this truck at around 325,000 it had the EGR, cam and reservoir tank replaced under warranty. Don't believe the injectors/cups have been replaced. I've had no loss of power but a very slight vibration up hills....always checking for bubbles into the fuel tank but nothing...

Thoughts?

 

Thanks for the input......not good news. I did see something one time that it could be the transmission cooler (I shift) also....

 

Great idea....thanks...