Mystery leak - help identifying

[alvocado]

2011 GMC Yukon 5.3L, 135k miles. I have a mysterious, intermittent leak that I can't trace and am looking for some help. I first noticed some dried, brownish dust on the garage floor which looked like a dried fluid. It was powdery and didn't leave a residue. This happened twice over the summer. While changing the oil, I noticed similar colored residue on the steering rack and adjacent transmission cooler lines (see pic.) The picture shows the rack near the boot on the passenger side of the truck. I can't see any fluid or residue above the rack. I checked the head gasket and the front half is dry; I could only see  the front half due to the exhaust manifold blocking the view from above and I couldn't get a mirror on the back portion. Some other notes:

• I'm not losing any coolant. The level is constant in the reservoir
• I've had a strange popping noise at cruising speed for the past year that has been elusive. It doesn't vary with RPM and is random. It sound like something flapping under the track. It may be unrelated but I'm noting in the event it's exhaust related or something that could happen with a manifold or head gasket issue.
• The truck seems to be a bit louder than usual at highway speeds under hard acceleration

 

Any help is appreciated.

[davester]

that could possibly be from the a/c drain tube, depending on where it's located and what it drains onto...

[alvocado]

8 hours ago, davester said:

that could possibly be from the a/c drain tube, depending on where it's located and what it drains onto...

I did check the A/C drain and it's very unlikely to be the cause. The tube is located far enough away and I couldn't see a way for condensate to work it's way over to this area of the steering rack. 

 

Another thought after posting is the heater core or heater lines since those are at the firewall. I'll inspect those today.

[alvocado]

All heater hoses and connections are dry. I did notice that the engine mount directly above the stained area on the steering rack also has the same dried residue so any fluid leaking appears to be coming from directly above the mount. I can't see anything above the mount due to the exhaust manifold and shields. 

[Thomcat]

.02  Looks like the dried fluid is trailing down from between the two steel lines (tranny cooler lines) at about 10 o'clock from center of the photo and probably blowing onto the rack where it drains down onto the floor. Might want to follow those lines upwards off the photo to see if you can locate the source.