Word for sighing through your lips so that they flap and make a silly noise

If you take a deep breath, close your lips gently, and then exhale, the air forces itself past your lips and makes them flap together like a horse. Is there a commonly used word or idiom for this?

Paragraph 4 of this ipoom.net article about stretching one’s vocal cords mentions the benefits of “[performing] lip bubbles or motorboats.”

Vocal coach Jaime Vendera also mentions “lip bubbles” and “motorboats” together in his book Raise Your Voice (via Googlebooks) and compares them “to a horse pursing its lips and blowing out air.”

Outside the world of singing and voice training, young babies are known for blowing lip bubbles and making motorboat sounds just for the apparent pure joy it brings them.
I imagine, however, that many babies rediscover and even expand upon the enjoyment of motorboating (as defined by Urban Dictionary) as they enter young adulthood, and for this reason I would recommend that you refer to this as “[blowing] lip bubbles” instead of “motorboating,” at least until the third date.

I think you mean a voiceless bilabial trill.

From wikipedia:

The bilabial trill is a type of consonantal sound, used in some spoken languages. The symbol in the International Phonetic Alphabet that represents this sound is ⟨ʙ⟩, and the equivalent X-SAMPA symbol is B.

...

Features of the bilabial trill:

Its manner of articulation is trill, which means it is produced by directing air over the articulator so that it vibrates. In most instances, it is only found as the trilled release of a prenasalized stop.

Its place of articulation is bilabial, which means it is articulated with both lips.

Its phonation is voiced, which means the vocal cords vibrate during the articulation.

It is an oral consonant, which means air is allowed to escape through the mouth only.

Because the sound is not produced with airflow over the tongue, the central–lateral dichotomy does not apply.

The airstream mechanism is pulmonic, which means it is articulated by pushing air solely with the lungs and diaphragm, as in most sounds.

We use this in choir as a warm-up, both with and without voice. We call it a lip-trill.