I installed a new y pipe today (120$ ebay) and everything seems to check out fitment wise except there is a slight vibration or rattle coming when you get on the throttle. And I even made sure all gaskets were nice and tight where the flanges are and there are still small water droplets coming from a flange, if anyone is or has had this problem pls get back to me. I'm think it's a leak or something.
I've also probably been driving with an air leak for close to 2 months now (plus however long the previous owner drove it since it's the stock y pipe I replaced) so would the ecu be adjusting to normal air flow now or something?

 

2 gaskets is probably not a good idea.

 

The problem with exhaust leaks like that is the flanges get all crusty with corrosion over the years, you need to make sure you are taking a wire brush to the mating faces of both flanges to get them really clean. If the gaskets are deformed get new ones, they take a LOT of abuse as well.

Here's Nissan part numbers and torque values, if you didn't torque the bolts I'm not surprised they're leaking. You local Nissan should stock a lot of these gaskets, they're all a reusable style but they do get beat to **** over the years and will need to be replaced eventually. Nissan recommends replacing them every time the system is disassembled.

Header to catalytic converters: 20813-AL500 @ 20 ft/lbs
Catalytic converter to Y-pipe: 20692-1E810 @ 40 ft/lbs
Y-pipe to mid-pipe: 20692-65J00 @ 40 ft/lbs
Mid-pipe to muffler: 20691-AM800 @ 25 ft/lbs

If you are still having issues getting it to seal up then spray both sides of the gasket with Permatex Copper Spray 80697. I always use it for exhaust manifold gaskets and head gaskets, especially on headers with thinner flanges that tend to move around with heat. Works great on all the other exhaust gaskets as well since you're dealing with unmachined surfaces usually.

 

So I checked and sanded everything good and put it back on but it was still rattling at 2k plus rpms, rattling so bad shakes entire seat. Got home and checked to see if there were any more air leaks this time which there wasnt, but it looks like part of the y pipe is rubbing on the frame above the flange.
could the rubbing really be that bad or did I put this **** on upside down like a dumbass if that's possible

 

So I checked and sanded everything good and put it back on but it was still rattling at 2k plus rpms, rattling so bad shakes entire seat. Got home and checked to see if there were any more air leaks this time which there wasnt, but it looks like part of the y pipe is rubbing on the frame above the flange.
could the rubbing really be that bad or did I put this **** on upside down like a dumbass if that's possible

 

There's definitely an up and a down side to the y-pipe. The OEM one all the bolts studs for the heat shields face UP I think. I didn't think it would be possible to put it on upside down because there's a slight bend but MAYBE YOU CAN ???

 

Checked out the area I thought was rubbing and sure enough the pipe is pressed up super tight against the bottom of the frame which is making the crazy vibrations. Maybe bend the pipe in slightly at the point of contact?