I've never been one to feel every let down either. Sometimes it's a pulling, tightening, tingly sensation ... more so on the breast that isn't being nursed on. I relied more on the swallowing pattern of my little one changing to long and drawn out sucking, many times I can actually hear the milk being swallowed when I let down also. And also she will sometimes make this little puffing sound as she swallows.
This is what kellymom.com says about it:
It is normal for let-down not to feel as strong as your baby gets older. Some mothers never feel let-down, and some stop feeling the let-down sensation as time goes by. This does not necessarily indicate that let-down is not taking place.
Reliable signs of a healthy, functioning let-down include:
In the first week or so, mother may notice uterine cramping during letdown.
Baby changes his sucking pattern from short and choppy (like a pacifier suck) at the beginning of the feeding to more long, drawing, and rhythmic a minute or so into the feeding.
Mother may have a feeling of calm, relaxation, sleepiness or drowsiness.
Mother may have a strong sensation of thirst while breastfeeding.
Baby is swallowing more often. A swallow sounds like a small puff of air coming out the baby's nose and you can usually see the muscle moving in front of the baby's ear, giving the baby the appearance of his earlobes subtley wiggling.