I could not see how 喝 here means drink, so I tried to look further in my dictionary and it says:
The same as 嗬, which is an onomatopoeia
Which kind of makes sense since I also found a similar expression: 哼了一声.
However the dictionary seems to give it a tone of surprise, but in that case what does it mean together with 低? Maybe I could translate the sentence as:
he made a small oh