Or hum one, if that's your preference. It's mine, since my whistling skills are less than stellar. It's all music, and more importantly, it's your own music. It's you making music for your own pleasure. It's something you can do any day, any time. And it's really good for you. Good for your heart, don't you know? And since you're doing it for yourself, cavil me no cavils about how awful your ability to track a tune is. Even if you lose the tune in the first bar, even if you can't carry it in a wheelbarrow, you go ahead and make music. Do it in the privacy of your own home if it makes you feel better, but do it. Lather, rinse, repeat.
And if you have trouble figuring out just what it is you want to whistle, hum, warble or croon, turn on the radio. Listen and sing along to whatever you hear there. Doesn't matter what music you're hearing, or whether it has words at all. You can whistle along to classical, you can warble to jazz. (You can even sing along to songs you hate. Think of it as being your way of improving their quality.)
One place I turn to when I want to sing along with everything in the world, all in one afternoon, is Hober Radio on the 'net. You may already know about Hober. If you don't, give it a listen, if you're in the mood for eclectic folk, Miles Davis, didgeridoo (lots of didgeridoo. It's a Hober thing),and lot of uncategorizable stuff, too. Check it out, and let me know what you think.