I've got the same noise. It happens usually between 3-4k RPM whether I'm accelerating slowly or quickly. At first I thought maybe it was valve noise but it doesn't happen all the time, probably 70% of the time. And like you say, when one thing goes wrong, others do too. I got the CEL last week with the PO420 code (catalyst output beyond limits or something). Auto Zone cleared the code and it hasn't come back yet. I would love to know what's causing that noise though!!
