Originally Posted by
Merlin086
The harmonics within the car are way down, it appears it was the header pipes vibrating against each other underneath the crossmember. A lot better outside too now, must have been also affecting the sound level at the mufflers.
Should be heaps better with the final production headers which are 15mm longer on the vertical to clear the crossmember more, as mine are vibrating occasionally depending on revs and gradient.
Just did a accurate check of a tank of mixed highway and town driving, averaged 8.2L/100k's.