black sludge in coolant

[Jack hward 360]

Hi there, I own an Audi A4 b7 2.0L TDI, head gasket went a month or so ago, got it fixed, a month later, I’ve noticed that the car has a black sludge in the coolant tank, as well as this, it’s been starting/ idling about 200/ 300 rpm higher than normal (900 rpm), this usually occurs at the same time that I hear a rapid fire knocking coming from under the dash of the car. 
 

i have googled it and forums suggest it’s a oil cooler leak, and I need to get that fixed. 
 

From my info, does anyone have any thoughts about what might be going on with the car?

just to add. The engine warning light just came on, with an Orange light! 
 

thanks, Jack.

[Jack hward 360]

Let me know if anyone wants more information.