How about a simple chant on one, two or three notes, accompanied by a simple bongo drum beat (or similar) – or the congregation could have various percussion instruments.
It is easy to pick up the rhythm of the words from Acts 1 and anyone with a bit of musicality could lead it:
“You will re-ceive po-wer when the
Ho-ly Spi-rit has come u-pon you,
and you will be my wit-ness-es…
to the ends of the earth.
(Repeat several times, getting louder each time). Allow silence at the end. This chant could be used instead of a hymn before the Gospel reading, for example, or near the end of the service.
Having used the chant, it would make an evocative beginning to your celebration of Pentecost, too!