Hi!
Hooting is in most cases just from a slightly blocked nose. Unlike humans, guinea pigs can't use their mouth much for breathing so any obstruction will sound much more dramatic than it is especially as guinea pig airways are very small and tight. Hooting for a short time twice in a day is nothing to worry about. The offending particle will be in most cases be simply sneezed or breathed out again.
If you worry about a respiratory infection, please be aware that the common bacterial URI is usually only seen in piggies with a lowered immune system through stress, illness, bereavement or because it is not yet fully developed or in older piggies very frail when they are in close company to an already infected piggy (you will recognise that pet shops are combining several of the risk factors!). Fully healthy companions can fend off the bacteria on their own and don't usually catch it. I would really not worry after this time!
However, if you see any serious symptoms and a fast deterioration (loss of appetite, lethargy etc.) then you need to see a vet promptly as you are dealing with some of the rarer nasty respiratory illnesses/pneumonias.
The breathing noises associated with URI are crackly or raspy breathing from the throat or lung area as well as clicking in the lungs in the case of pneumonia.
New piggy problems: URI - ringworm - skin parasites
I hope that this helps you? Please take a deep breath!
