Jake Brake Issue

I have had my truck in the shop twice now for a Jake brake issue. Both times they have told me that they "ran it up and down the hill, and nothing is wrong"." It is a company truck, a 98 Volvo 670 w/ Cummins Engine. Originally, the issue was that occasionally the jake brake wouldn't activate. I usually drive with the jake on so it will automatically kick on when I release the accelerator. I could either flip the switch off/on or depress/release the accelerator again and the jake would kick in.

Now, it still does that (so rarely that I can't seem to replicate the issue with any regularity or on demand), but also it has begun to have an added "grinding" noise that lasts about half a second before the jake kicks on. It doesn't happen when the jake is turned off and I release the accelerator, but does happen if the jake is on and I release the accelerator, or when I have already released the accelerator then flip the switch on. Sometimes there is a delay in the activation of the jake break (2-3 seconds) which is obvious because of the lack of deceleration, combined with the "grinding" noise that happens later than normal. Again, this part is intermittent, and I can't replicate on demand, it just happens. But the noise is there EVERY time. I have tried to replicate the noise at all 3 states (low, medium and high) of the jake and it happens on ALL levels (I was thinking it might be a certain stage that was faulty)

Any ideas what could be the problem? I would like to be able to point the shop in a direction.

 

I will give that a try, thanks.