Its probably the serp. Belt slipping. My 11' 4cyl does it too going into 2nd and 3rd if I'm on it hard 7000-7200 and upshift hard. Sometimes it'll bark the tires other times the gear change is too much of a rpm difference and it will make the belt slip.
With that being said I still continue to do it and I'm sure its hell on the timing chain and guides. I probaly should have bought a v6 this things a dud, but I get great mileage from it so it'll have to do.
56k on it now and it gets 100 more almost everyday here in nyc