A Tenor cricket in the tranny?

[Ron Brooks]

Out of the blue, my 88 300SE is making an odd sound for the past 4 days. If I coast with my foot off the gas entirely between 45 and 30, there is a continuous high pitched chirp like a cricket hitting a high note. If I shift into neutral it goes away instantly. If I touch the gas just slightly it goes away instantly. The noise is not effected whether I am turning either direction or going straight.

The only thing I can point to in those days was that my wife filled the car for me, but she put regular (87 octane) in it. We spoke and she'll be sure to use 93 (lowest available after 89 here in NY) from now on. Now, the car does not ping at all and I can't see any way the gas could be related since my noise is when coasting.

Any ideas? Could this be from a vacuum line somewhere? It's fairly loud.