2004 Toyota Solara 57,600 miles V6 automatic

Hi, I brought my car to a new toyota dealership today (we moved so its new to us) for 60k service, I did so early because the winter weather is coming up and they'll be swamped, also, a few weeks ago I began hearing a rumbling sound underneath the car loud when I excellerate. Its annoying but doesn't seem to effect performance. I assumed that it was the muffler although when I had the brakes done they said everything else looked good (3 months ago) The service tech at Toyota told me that everything is fine, the noise is coming from the catylitc convertor but its nothing to worry about. My bill was $438, No brake work needed they were newly installed, they didn't rotate the tires because my best are in the front and I've been told I need new ones before winter (by them). I left there feeling bad, I don't like the sound coming from my car, if its the CC its still under warranty, I think they should fix it so it doesn't make the noise, am I being unreasonable in your opinion?

Thanks so much (inadvance for your help)

Teri