No, it wasn't a silent whistle. Everybody could hear it, but my lab was the only one that seemed interested. It looked just like this:
Since he already had good voice recall, my plan was to take him out in a big field, let him wander off, then call him to me, blow the whistle and give him a treat when he came. (If he didn't have reliable recall already, I would have used a long training lead so make sure he wouldn't ignore me.)
The reality was, after he wandered off and I blew the whistle, he came charging back with so much enthusiasm that I was afraid he would run me over. Somebody had already whistle trained him.
After that, I would sometimes reward him with a treat when he came to the whistle, but usually I would just tell him (under my breath, if there were other dogs and owners nearby) that he was the best dog that ever lived and I was grateful that he decided to join our family.
Then I'd scratch his back where he couldn't reach. (My training techniques are not terribly sophisticated, which is why you'll rarely hear me offer advice on training.)
My current dogs are not whistle-trained, but we're working on it.