The tinsel leads to the VCs could have potentially been torn away, where pressing in the cone would reestablish a connection.
Try: Remove the sub from enclosure, check wiring, if it looks secure, check the voice coil leads are not shorting against each other or the speaker frame. Then, with the stereo off, see if the speaker moves free if you push gently on the cone. If it is stuck or scrapes, the speaker is bad.
I couldn't get a hold of my friend's multimeter, but here's a video of what's happening. It sits still until I push it in, at which point it starts playing. There is a slight rattle to each hit, but it's not nearly as bad as the video makes it sound. The mic on my phone is just getting maxed out because it's too loud.
Video Link 1
Same video (youtube)
Possibly pushing the sub too much? Going to have to try the process of elimination.... Try a new sub... see if it does the same thing.... try the current sub... on a different amp.... Is your amp set to LPF? try new RCA's hooked up to a different deck.... see if you get the same result..... basically you're going to have to isolate and test each part of your system.... only way I can really figure on seeing what's wrong..
I know it's not the amp/deck/rcas, since my front speakers are powered off of the same amp and they have no problems.
Just because your front speakers work... doesn't mean that the amp isn't bad.... could have a bad output on the sub channel than on the front speaker channel... it almost sounds like the LPF is gone... you really should borrow a multimeter an just check the sub.... and make sure you disconnect it from the amp too when you meter it... because sometimes the amp can throw of the reading... good time to pull the sub out and check the wiring...
The sub has been in and out twice since I've started troubleshooting, and it's currently out. I strongly think it's the sub, since touching it is what makes it work. I have had this happen on a Dual brand sub a friend gave me a while back. It would work for a while if I pushed it back in, but after a while it completely gave up. This was powered off of a completely different amp. I could try hooking the fronts to the sub channel to see if it works, since I need to turn the high pass filter off for now anyway.
Grabbing another sub isn't exactly easy as pie, since I have no other friends with systems in their cars, and I don't just have piles of speakers lying around. The fronts and sub are powered off of the same amp, but the sub that blew a while back was powered from something separate, so I know it can happen to a sub independent of what the amp is doing.
I know.... Does the sub make a scratching/scrapeing noise when you push in on it... without the amp running... I had a sub the other day that someone allowed to be returned, without being tested, and then it got resold as an open item... yah.... it was ceased up, heck you wouldn't of even had to test it, just push on the cone once and you would of known it was crap..., and the guy that did the return was too stupid to have it checked.... Guy ended up getting a much nicer sub for the same price, because the one he had picked out... we were out of stock on...
The wiring is good, but I did notice a tiny bit of residue (with an oil or grease consistency) on the back of the cone where it would possibly come into contact with one of the VC leads. The cone moves freely in and out if I push it without scraping or anything.
Covered that one.