any ideas

The heat shield on the muffler is tack welded on in four places. The shield has a propensity for rusting and cracking and otherwise generally becoming loose, creating a rattle noise at certain rpm's when resonance occurs. This is often reproducible just by gently revving the engine when sitting still or by whacking on the muffler with a rubber mallet.

Sorry, left this out of the previous message. If it is the muffler heat shield, don't worry about it unless the heat shield actually comes off. Or, strap it on with a huge hose clamp. While the shield is there, the noise is nothing more than an annoyance.

Once the shield is gone, you must replace the muffler. The added heat will melt the fuel door release cable plastic which will lead to rusting and binding of the cable.

Thank you so much! I will take the info to my mechanic. Liuly Mae