help diagnosing thumping noise under car

[m35x-2]

What makes this noise difficult to figure out is it come and go. I heard a double thumping noise 2 weeks ago, turning into a parking lot. It sounded like its coming from the back. I thought it was my bass tube rolling around. I didnt think nothing much at the time until on Christmas day when I was on the highway. I was going around 70mph and I notice the rotational thumping noise while switching lane or in a slight turn. I slow down to around 60mph and the noise slow down too.
I got off the highway and the car was driving fine. I made left and right turns and there were no noise. The problem is intermittent and I know that will make it more difficult to diagnose.
I pamper the car and only drive it on the weekend. I don't have any oil leaks. Its cold outside but tomorrow I will try to go under the car and inspect. I plan to inspect all axles and drive shafts. I will drain the rear diff to see if any metal comes out. What else should I check while I'm under the car?

[Ace2cool]

Diff bushings. Subframe bushings. If it has a 2 piece driveshaft, the center bearing may be busted. Jack it up and put it on jack stands, then as safely as possible, just start trying to shake things. If it's not supposed to move and it does, you've found your problem. Don't just try to shake axles up and down, but lengthwise as well.

[m35x-2]

thanks for the advice but I didn't get a chance this weekend to go under the car because of the bad weather. I will post what I find when I check it out.

[m35x-2]

Diff bushings. Subframe bushings. If it has a 2 piece driveshaft, the center bearing may be busted. Jack it up and put it on jack stands, then as safely as possible, just start trying to shake things. If it's not supposed to move and it does, you've found your problem. Don't just try to shake axles up and down, but lengthwise as well.

I went under the car and everything looks ok except for some small amount of grease on the right front axle boot. I also went on a 200 mile road trip and didn't hear any unusual noise. While I was on the road trip I notice that my gps that was mounted on the windscreen was vibrating above 75 mph. Since the steering wheel wasn't vibrating I will assume the rear wheels are out of balance.
I came to the conclusion that the 2 sas bazooka tubes that I have in the trunk that is not secure were hitting each other. I did mention that I was hearing the noise on the highway while in a turn. I feel centrifugal force had the two tubes against each other and the out of balance rear wheel was causing the tubes to bounce against each other.