Tough gremlin to figure out

Hi all, I have a 07 king ranch that I recently purchased second hand. So far, things I did was a EGR delete w/new oil cooler, studs, gaskets, serviced all fluids, flushed cooling system. All new filters (motorcraft) re-soldered ficm (it actually worked lol) straight pipe exhaust and sct x3 with custom SPD tune (regular lock up) from Chris B. over at truck source diesel. My problem is something I can't find anywhere on the internet. So lately my truck exhaust note has become quieter. When I get at it wot , it doesn't have that famous straight pipe sound. It almost sounds like I put a muffler on it. You can still hear some rumble, but not the loud rattle that wakes up the neighborhood. Another thing is some days, the sound comes back and sounds fine. This recently started happening. I've been driving around with mods stated above for about 3 weeks before problem accured. No codes, no loss of power or boost. Only thing I notice different is that when I open her up, and let off the throttle, I hear almost like a pshhhhhh sound. I guess u can say kinda like a wastegate but not a turbo fart. I'm fairly new to a powerstroke so please any help will be appreciated. Don't worry about my name and sig cause that's my older truck. Thanks!