Strange Intermittent Buzz/suck/scrape Sound from under the TI - Not Constant?

[atlanta318ti]

Driving down the highway at lunch today next to the median wall, I heard the strangest sound. I was cruising at about 80 with the window down (65 degrees today in Atlanta). RPM was @ around 3500 and constant. And all of a sudden I heard what I can only describe as a Buzzzzz/sucking/scraping sound. It lasted about 20 seconds, then stopped for 20 seconds, then started for another 20 or so and I didn't hear it again. Since I was next to the median wall w/the window down it was amplified. It may have done this before and I just never heard it.

I literally have no way to describe the sound. It wasn't overpowering or extremely loud, but it was noticable almost like an electric belt sander. It was louder than the sound I might normally hear from the ASC-T (which you can hear adjust in the engine compartment when it's needed and feel slightly in the brakes) and I didn't feel any change in the engine speed or brakes. It wasn't as loud, on the other hand, as a rotor grinding. Then the strange thing is that it almost sounded like something was trying to escape from backpressure? No vibrations in the car or steering either.

First, has anyone heard or experienced anything similar to what I'm describing? If yes, any input is greatly appreciated and you can ignore what I've written below.

Secondly, below are the only other symptoms I've had lately that I'm getting ready to repair although it didn't seem related to this are the following:

1) changing camshaft position sensor (on and off with CEL) this weekend.
2) recently changed front pads and rotors along with right front ABS/ASC sensor due to light. (ABS/ASC light hasn't come back on since I changed the sensor a month ago).
3) added new tires the week of christmas (Goodyear Eagle F1s stock 16" size 225/50/16).
4) Although there's no loss of RPM at high speeds, it feels like there's a lot of backpressure above 65mph and I'm fairly certain my Catalytic converter shield or the CAT itself is ready to go (rattling briefly around 2-2.5k acceleration).
5) Shocks/struts (originals with 147k) are starting to go so I'm getting ready to order in next 2 weeks. Bounces and bottoms out on speed bumps. Starting to hear some movement in the mounts on roads that shouldn't be a problem.

[mooseheadm5]

Could be the cat? Hard to tell without seeing the car. You can check to see if the guts are loose by supporting the car and banging on the cat with the side of your fist (make sure it is not hot, duh.) Sometimes heat shields will buzz as well , maybe yours was at just the right resonance at 3krpm to do this in this pattern? Bad news is that if it is the cat and it is OBDII, you are looking at $$ to replace it. Cheap aftermarket cats do not last on these cars, so we don't even sell them (not after we replaced 3 under warranty on a 318i sedan in one year.)

[atlanta318ti]

Thanks. It's OBDII. I need to get it up and check it out this weekend. Could that also account for the feeling of backpressure on the engine? Normally I believe that reduces HP, which I'm not feeling, but thought I'd ask.

I hope that description makes sense. The only better way to describe the backpressure is like driving on 3 cylinders but without any loss in horsepower. There's no check engine light on, and the car idles fine so I assume I'm still running on all four cylinders and I'm only able to compare to my prior non-sport TI. My first 97TI felt like it had a lot more headroom (or less torque) above 70mph. Although I can step on the gas at 70-80 and torque is unreal, it feels a bit too tight.

[bbbmw]

My original catalytic converter started the "canful of rocks" noise at about 20k miles. It did the same thing you're talking about, without any real difference in driveability. Coincidentally, it started making a rattle noise last weekend at about 2k rpm. I haven't had a chance to get under it, but today the "check engine" light went on. Am thinking it is the catalyst again.