The sound only happens when the front wheel is going up a curb, speed bump, etc and the passenger side front suspension is extending.
If you have done everything else already which it looks like you are going to have to remove the spnidle and check the shank of the ball joints. I replaced over 50 sets of ball joints In Arizona with sn197 style mustangs 2005 and up.
If you spoke with Griggs or Jason at Maximum Motorsports they are going to come to the same conclusion. You can't correctly check ball joints like you did in the old days because those we're made out of metal and these are made out of hard nylon and they will give you a false reading while being unloaded. I'm not saying if really super worn out you wont be able to get movement out of the wheel while being on a lift or jack stands. The ball joint isn't really bad in that sense but it has groves in the ball part or it and pops on the race.
That's why they tell you not to tighten the sway bar end links with an impact gun because you will spin the nylon ball causing grooves and you will get popping out of the end links also.
I promise you they will be bad ..meaning when you press on the shank and let go they will come back to center and not stay where you position them with the correct amount of tension on the shank.
If you put some Steeda x-5 in with the taller shank you wont have anymore problems and they have a grease fitting ..Moog makes these also in the stock shank size with a grease fitting.
Lowering springs really eat up these ball joints for some reason, people are ridding on bad ball joints and don't even know it. This is my step by step procedure.
1) I check upper strut mount ..make sure it has gt500 mount, updated TSB mounts,or camber plates.
2) I remove sway bar and end links
3) I check hydro mounts on A-arm and the other busing on the front of the A-arm and make sure all bolts are torqued to spec
4) I check to see If front strut is damaged or leaking and the bolts are torqued to spec.
5) I check k-member bolts and torque
6) I check front brake calipers bolts
7) I check tie rod end nuts are torqued to spec.
8) I REPLACE BALL JOINTS !
I will tell you something you don't have to remove front A-arms to remove BALL JOINTS , get your self a C-cup remover and press them out . I can do each side once the tire is removed in 30 minutes.
