flapping noise on acceleration

[Jt525i]

hey everybody my 94 525i standard transmission has been making this fairly odd noise that sounds like a card in the spokes of a bmx tire and i feel a thud on the bottom of my car if i floor it right away when the noise is being made. at first it went away when i let off the gas but now it is starting to do it almost all the time it starts to go away in third gear when im going faster. the noise seems to be coming from the right rear in the driver seat and the left rear in the passenger. a couple days ago when i was bleeding my clutch i noticed that my transmission mounts are fairly weathered and cracked and it sort of wiggled a little when i banged it with my hand another thing i noticed is that my guibo has threads coming out of it and that it is also cracked and weathered . (driveline has zero play felt pretty sturdy) maybe thats the thud and the guibo is the noise? idk i am very confused as to what might actually be making that noise and i would like a little help. thanks.

[Doge]

In some ways it sounds like a Center Support Bearing (CSB).
Over time, and especially if the trans mounts and the drive-shaft guibo are showing signs of dry-rot, the bushing material (rubber) that holds the CSB captive is probably torn out causing the bearing to wobble and hit the various points of mounting.

Park, in neutral, rev engine to various levels. Then drive the car at those levels and compare the feelings...If no presentation during the neutral reving but while driving then very likely CSB...if presentation during both then might be trans and engine mounts.<--mind you this is not a "truly" definitive test.

The best way to know is to drop the exhaust, drop the heat shield and inspect the drive shaft. The CSB should have NO play aside from minor deflection of the rubber surrounding the bearing...but it sounds like a CSB. Put your hand up against the tunnel when the "flapping" happens, this can help indicate too.

EDIT: I know you stated that you checked the drive line...but did you pull everything down so you could see the entire thing? The whole drive shaft? The shaft is pretty hefty and might not move enough from one end of applied force. Levers are funny that way.

[Jt525i]

In some ways it sounds like a Center Support Bearing (CSB).
Over time, and especially if the trans mounts and the drive-shaft guibo are showing signs of dry-rot, the bushing material (rubber) that holds the CSB captive is probably torn out causing the bearing to wobble and hit the various points of mounting.

Park, in neutral, rev engine to various levels. Then drive the car at those levels and compare the feelings...If no presentation during the neutral reving but while driving then very likely CSB...if presentation during both then might be trans and engine mounts.<--mind you this is not a "truly" definitive test.

The best way to know is to drop the exhaust, drop the heat shield and inspect the drive shaft. The CSB should have NO play aside from minor deflection of the rubber surrounding the bearing...but it sounds like a CSB. Put your hand up against the tunnel when the "flapping" happens, this can help indicate too.

EDIT: I know you stated that you checked the drive line...but did you pull everything down so you could see the entire thing? The whole drive shaft? The shaft is pretty hefty and might not move enough from one end of applied force. Levers are funny that way.

i only wiggled the part of the driveshaft i could see and it has gotten much louder and consistent as now it does the noise when it wants to. whenever i go on the freeway in 4th and 5th gear the shift stick seems to vibrate and create a rattling noise and the car seems like its trying super hard. i feel the vibrations and bumping coming from the foot divider in the rear seats. the bumping and flapping noise seem to be the most prominent when picking up speed in first and second. also how hard is it exactly to drop the exhaust and heat shield ?

[MassiveWalrus]

how hard is it exactly to drop the exhaust and heat shield ?

Super easy IMHO