turbo noise or exhaust leak?

[JimT]

well, things have been pretty good for a while now with no major engine issues at a little over 200,000 miles since my inframe, 789,000 total miles on this 2012 Pete 587 CM2250. So it's not entirely surprising to see one pop up now. Just yesterday while driving to my delivery I started getting an odd metallic whining sound under hard acceleration only. Hard to describe exactly it sounds kinda like the whine from a bad power steering pump on a car when your turn the wheel all the way. Or an electric motor under a heavy strain. Clearly from the right side of the motor it only occurs under hard acceleration and is not tied to boost pressure. If I slowly get into the throttle I can get the boost up to at least 28psi with no issue. I didn't try going higher at the time since I was driving first and foremost, loaded, and not looking to break down before understanding what's wrong. So I took it easy, delivered my load, cancelled today's pickup and got home just before dinner after a nice 14 hour day.

The turbo was my first suspect and after some searching on here I decided to check for exhaust leaks before jumping to conclusions. Well I found them, but not at the turbo mounting gasket as I thought. No, I found 3 broken exhaust manifold bolts with only the slightest sign of soot around the #1 exhaust. #1 had both top and bottom bolts broken while #6 had just the bottom broken. That's gonna be a fun job... not.

Replacing bolts is one thing and if that's all I need to do I'll be thankful. But what I'm wondering is why they broke. Is there something else I need to address? I don't run hard. I rarely push boost levels above 21-ish psi, even loaded uphill and I operate almost exclusively between 1450 and 1800 rpm with the low end only for flat cruising at low boost. The only thing I did differently immediately prior to the whining was allow the engine to wind up to 1900 at about 18psi boost going up the hill on I-84 east just pas the PA border. Then I stayed in gear using the engine (not the engine brake) to limit my decent speed. Engine temps never went above normal and the fan only kicked in once or twice during the climb which is typical for the conditions.

I don't know exactly when the bolts started to break. Last time I thoroughly inspected the exhaust manifold was a month or two ago when I replaced the IMAP and exhaust sensors. It's possible at least one was broken at that time and I missed it, but I know for a fact that there was absolutely no sign of an exhaust leak at that time. The area around the manifold has been exceptionally clean since the inframe.

So, in absence of feedback from you fine folks, my current plan is to replace the broken bolts and put it back together to see if that solves my issue. Am I on the right track? Is there anything else I should do?