Consistent hollow hitting noise behind firewall/floorboard......
Hello everyone, first sorry for the vague description in the title it is just hard to describe. So first I have an 08 F350 6.4 and when I sit at idle maybe every 30 seconds or longer I hear this hollow noise, almost like someone taking a small mallet and hitting under the floorboard near the passenger side or the bottom of the fire wall. Almost like a valve that is maxing out for a split second. So the only thing I have noticed on my SCT Livewire is that the exhaust back pressure will go from 119 in. hg to about 121 for a split second when I hear the noise and then it immediately recovers.
So, any help, ideas or insight will be greatly appreciated!
BTW, no trouble codes.