Usually that sound is a loose smog pump connection. Do you still have the smog pump? There are multiple hose clamps on all the hoses runing from the smog pump to the blow down tube that goes to the mid pipe (in your case your off road h pipe. It does not need to be used, as it blows on the cats, and since you have none, there is no need) The offroad h pipe I had on my car when I bought it didn't come with that connector to hook the tube back up, so the previous owner put a bolt in that hose on the bottom off the valve and clamped it in. Anyway.. just check all those clamps to see if any are leaking.