1st Gen R53 Cooper S My MINI sounds like a Subaru!!!

I have an '05 R53 with about 156k miles on it. I have some basic engine mods (15% pulley, plugs, wires, coilpack, intake & exhaust). Today, I was driving home from work when all of a sudden, my MINI started sounding like a Subaru. It was driving completely fine. No hesitation, no power loss, no check engine lights. Nothing out of the ordinary, except what it sounded like. I felt like someone just switched my motor out with a boxster engine from a WRX. I drove it for another couple minutes like that, checking the gauges on my Torque app and everything appeared fine and normal. Then after about 5 minutes of sounding like that, it stopped. Just as fast as it started, it ended and it went back to how it normally sounded. It was the weirdest thing. I don't really know what to think of it. What do you guys think happened?

Like I said, it ran perfectly fine like that, no loss of power, no hesitation, no check engine light. I stopped at a convenience store after the sound stopped. It turned off and on without any problem.

I'm pretty stumped about it. Have any of you heard of something like this happening before?

 

What do you think it could be though? I'll check everything I can, just trying to figure out what this could have been.

I have a data log from when it happened. It unfortunately doesn't show much besides the boost, throttle position, speed & rpm's.

https://docs.google.com/spreadsheet/ccc?key=0AiKzLEKSLWsKdGpwbDNVQkozbjlXRE9DV2o5V29UTXc&usp=sharing

Not sure if that'll make a difference or not, but just thought I would share that as well.

 

I have a cat back Group 4 Autosports exhaust. No muffler, no resonator. It's basically a straight pipe with an inline "bafflepack" and "glasspack" and it's been on my car for about 5 months (6k miles) or so. I swapped it out just before MOTD.

It had the same problem this morning when I started it up. Sounded just like a Subie again. This time I could hear a light tapping from the motor. Popped the hood and I could hear it. Sounded like a valve tapping the piston. Still ran fine performance-wise and after giving it a couple decent revs, the taping stopped along with the Subaru sound. I'm thinking one of the exhaust valves might be sticking and not closing all the way, hence the tap and possibly the Subie tone from the exhaust. If a valve isn't closing all the way, but still closing most of the way, I would expect this could cause the sound without a noticeable amount of power loss. What do you guys think?

I will see if it sounds like that again when I start my car later today and I will try to get a video of it so you can hear it as well.

For the most part, I don't beat on my MINI. I do rev it out every once in a while but for the most part, I keep it under 4k rpm's on most days


The other idea I had was the upper cat is starting to fall apart and little bits were getting stuck in the lower cat, but with hearing the valve tap this morning, I'm guessing a valve job is in the near future....