I'm not sure if there's a better place to post about this, but this is the general forum so maybe I got it right. Starting a week or two ago I noticed a weird hissing noise when I was on the highway. It seemed to be paired with turbo spool and sounded kind of like an exhaust leak or something. I got under the truck and made sure the v band clamp that connects the down pipe was tightened all the way and it seemed to go away after that. So I thought I had fixed my "exhaust leak".
But, a couple days ago, it came back. Today it was especially noticable, but it definitely isn't linked with turbo boost. I tried giving the truck some more pedal on hills to spool it up a little more, and the sound wasn't there. I tried slowly building boost to see if there was a certain pressure, sound didn't correlate to that either. The only thing I know will make the sound get louder is when the truck shifts gears (specifically into 4th at about 30MPH and 5th at about 45MPH), the 68RFE does it's stupid RPM flare on the shift and the exhaust hits a different note. That specific thing seems to make the sound very loud. Gear shift followed by RPMs changing and the exhaust hitting a different note makes this hissing noise especially loud.
I also noticed that when it's doing it around 45MPH I can make the noise get very loud by giving the truck just a little touch of acceleration. When I'm doing that I noticed this: 43MPH - hardly notice, 44MPH - start hearing the noise over the exhaust, 45MPH - the noise sounds like a second turbo is spooling under my truck almost, 46MPH - starts fading away and quickly becomes unnoticeable again.
So....what in tarnation is going on? It isn't an exhaust leak that I can tell, I checked the clamps on my intake filter all the way down to the turbo, I checked all intercooler related clamps for tightness in case it was a boost leak, and I'm not losing boost or power when the noise comes around. I've heard power steering pumps make an odd noise on their way out but it doesn't matter if I'm going straight or taking a turn, the noise still comes around. The truck is also at 18K miles so I hope it isn't a power steering pump. If anyone has any ideas please give me some stuff to try and look at/fix. Thanks, and I apologize for the long winded post. I'm just trying to give every detail I can
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