Loud lower frequency sound at idle and when stepping on gas? Error code 51

Hi,

I've got a 1995 geo metro, 150k miles, original owner. It's been perfect up until the past few days. The engine light came on and I'm having issues trying to diagnose the problem.

The code is 51 (using that fuse trick).

I yanked out the EGR valve. I put a vacuum to check the EGR and it'll actuate at <10 mm hg pressure. I also put the vacuum on the other end that would create the vacuum (not sure what the disc looking thing is called, but it has a small tube connecting it to the EGR valve). That disc thing does produce ~10 mm hg pressure when someone steps on the gas. However, the pressure is not held constant. It's only momentary - maybe 5 seconds? Something like that.

With the EGR out, I cleaned the EGR by soaking the lower "valve" end in carb cleaner (avoiding the diaphram). I tried to clean the intake and exhuast lines, but will try a hanger this afternoon.

Here's where the problem lies. I put it all back together and there's a low rumble that comes from the car. I can't tell what's wrong with it. It's a constant low rumble...

The Cat was changed last year (May 2008). It's under warranty so if it can be a cat problem, I'll take it back. However, reading the forums on symptoms of a bad cat, it doesn't seem like it. No sulfur smell or anything.

Please let me know what you think i should do next.

Thanks for your help.