What size are the bolts? I'm curious if riv nuts could be used In place of fishing the nuts.
Edit. NM I found the install pdf . 7/16". Can get grade 8 riv nuts at 1/2".
http://fasteners.sherex.com/viewitems/ind-rivet-nuts-american-style-rivet-nuts-ca-series/ca-series-steel-inches-
I may...
I'd start with the two big bolts that connect the subframe to the control arm. Two locations. If they are tight then the control arm wouldn't move front to back by hand.
Also you said "post alignment" it started. Possible that whoever did your alignment may have loosened and not tightened other bolts for other suspension elements such as vertical links, etc. I'd check them all.
So it Does it make the noise when you push the car forward or backward when in neutral?
Subframe bolts should be torqued to 129 ft lbs and 13 mm bolts for subframe bracket to 41 ft lbs.
Another tip: for the vertical links, be sure to lower the car with a jack stand under the lower control arm directly under the vertical link. There is a flat spot. I used a lift to lower the car down until the jack stand barely compressed the rear control arm in that exact location. This...
That stock rear spring will come out a ton easier if you loosen and lower the subframe on each side. Unless you use a spring compressor. I had to drop my rear hangers on my exhaust because the subframe couldn't drop past the 3" pipes. Spring just basically fell out once I did that.
I see some interesting things after you added the polyfill. At some frequencies the sound is less loud. That could be good or bad. A box that is larger than recommended can degrade sound. Would love to see the test with a little less and a little more polyfill. This would help determine how...