I was Googling Guinea-Pig Noises (DD likes to play them to the GP boys to see them react. They are interested for a nano-second, then realise the other GP isn't real )
When I was a child we had mainly sows. 2 of them used to "sing". Both sows were the only 'mum' pigs.
First time we thought there was a bird trapped in the house (sow was pg at the time, we thought it might be her in distress) but she was fine.
Then later she sang beautifully. Months later the other sow sang.
Like a woo woo woo woo continuing for 8-10 minutes.
No idea why and never the boys or the 'single ladies'