So I seen a few posts on this and read them but they don't seem to fit mine just right like "yea that's it" so I recently bought a 03 350z base model Stick 6speed and I'm loving the car coming over from Ls cars it's new and fun but when I bought the car it has its issues well first week I bought it drove it all week adjusted to it going nice second week I took it to the drag strip (dumbass) mistake #1 found out the car had a welded diff it showed no signs of one when I had bought it tires didn't skid on turns didn't sway on curves but anyways first pull on track went great ran a low 8 @80mph wasn't bad second pull I heard a clink sound from the rear passenger fender weld **** my self cause it was Loud got to the end check on the suspension every looked fine yanked on the tire trying to move it nothing seemed broken or loose drove it home that night 40 min drive 20 mins into the drive I hear a knocking sound it's not loud but it sounds like someone's knocking on your door repeatedly non stop it's not like loud but you can hear it and you know it's there well nwxt morning let the car warm up and drove just fine knocking was still there but it was fine till I got on the highway soon as I went from 4th to 5th gear and accelerated the car started to sway and I mean bad to where I let go till she caught herself and straightens back out but if I slow accelerated it doesn't so it as bad but if I try to hit it it sways back and forth I've asked friends who own a few they tell me I have a bad Axel or the bearings are shot or the control arms about to give but when I checked it again this time jacked up everything seems in order nothings loose or missing is this just a normal thing is the welded diff causing the swaying on hard acceleration if it is why does it really only do it in 5th and 6th gear from what I read on other posts everyone says an alignment or just ignore it but I don't like knowing something's wrong and when it finally goes it causes more issues then to begin with ill try to get a video of it knocking and a few pic of the suspension when I get off work

 

Could be so many issues

 

Indeed, could be many things. But my guess is you have a broken rear strut top bushing. You could have other component wear or failure, like control arm bushings, sway bar end links/bushings, axle problem, differently problem, or wheel bearing. It's really easy to remove the rear struts assuming you still have the stock setup (coil spring separate from damper).

To figure out wheel bearing issues, assuming there's no excessive play during a wheel grab test, I recommend using an auto stethoscope, rear end jacked up, engine running/idling in second gear. You can put the stethoscope rod on the rear hub and listen for bearing noises. This is somewhat dangerous, so make sure the front wheels are chalked and the jack stands are solid/stable. You can also listen to the differential for crunchies. These tools are cheap and really handy for figuring out odd noises.
Cheers!
-Icer