Hello and welcome to Apple Support Communities, Barry!
We understand you were testing the Find My feature from your iPhone and when attempting to play the sound to locate it, you only get vibration. We'd be happy to provide information on why this is happening.
Since you're sending the request to play the sound from the same iPhone you're trying to locate, it will not play a sound. Instead, it will play a series of vibrations.
However, if you had another device and from the other device you sent the request to play the sound for your iPhone, it will audibly alert you.
We hope this clears things up.
Have a great day!