ISX "whistling" and shut down

[CoroView Shop]

Update:
We have now replaced the low side and high side pump (at the same time after discussing it with my employer), and the situation now is that it ran fine leaving the shop (unloaded), I took it for a test drive and after about 5-10 minutes of driving, we're back to the whistling noise and stall. Cycle the key a few times to fill the fuel system and there's about a 2-3 minute run time. I did notice that there are lines running into and out of the ECM cooler. Is there possibly a check valve in there that may be stuck open and causing an excess fuel return situation, or has anyone run into a bad/prematurely failing high side rail relief valve? Seems odd that it ran fine for a few days after changing the relief valve and now back to this situation. Is there also the possibility that this could be an ECM issue not commanding enough fuel (maybe a reach, I know, but trying to consider all possibilities at this point)
Thank you!