The other day I was getting ready to leave my flat and I heard it.
A shrill, but tuneful whistle reverberated around the narrow streets of my neighbourhood in central Madrid.
I peered from my balcony, and, in the distance, there he was.
His name is Antonio, an elderly man, dressed in simple clothes, who grew up in a Spain that is far removed from the country we both live in today.
And he is practising a Spanish tradition that is slowly disappearing.
Antonio is an afilador, which translates into English as a sharpener. He thinks he is one of only five left in the whole of Madrid.
Brandishing two of my best kitchen knives, which probably hadn't been sharpened for going on 10 years, I ran down on to a silent, sleepy, Sunday morning street.
It was 11:00, but perhaps four hours ago the same street would have been throbbing with noisy partygoers, doing what Spaniards do best.
Suddenly, I felt a pang of guilt as a petrol-powered stone wheel on the back of Antonio's motorbike whizzed around, and the tranquillity of that Sunday morning was lost.
Then he set to work, holding each knife in two hands carefully passing it back and forth, a craftsman of a dying trade.
Days earlier, I had tracked down another sharpener 310 miles (500km) south in Seville.
Rafael Romero del Campo is 53 and lives in the working-class neighbourhood of Torreblanca.
It's a part of Seville that the tourists don't see.
Ironically, in such a poor neighbourhood where unemployment is rife, the trade of a sharpener is now a stable, if low, source of income.
"I have five children and four grandchildren, and this job feeds them all," says Rafael.
I ask him if he likes the job.
"The truth is there's no other work. I used to also work as a builder on a building site, but I lost that job."
Rafael's 30-year-old son, Jose Antonio also used to work on the building sites, which were plentiful during the country's construction boom.
Now he's unemployed, and unofficial work is sporadic.
"I get little work at the moment, but at least my dad is able to earn every day," he tells us.
Jose Antonio shows no desire to follow in his dad's footsteps but he is sad to see this way of working go.
"It's a shame… his dad taught him, and he in turn was taught by his own dad."
Door step service
For local businesses in Seville, the sharpener is a vital service that saves time and money.
The melody from Rafael's trademark whistle warns them that he is approaching.
Then we watch as fishmonger Miguel Paredes has his knives sharpened outside his shop's door.
"I don't have the hassle of taking my knives somewhere else," he says.
And it's a sentiment echoed by those at the Bodegon de Sierra cafe, in Rafael's neighbourhood.
"He doesn't charge much," says barman Angel Gutierrez, as he slices Spanish ham with his freshly sharpened knife.
"It's something we shouldn't lose. It's something we need," he says.
Rafael Romero del Campo started sharpening knives at the age of 13.
For the first three years of a now 40-year career, he travelled around Seville on a bicycle, using pedal power to sharpen knives.
He spent only one day at school and cannot read, or write, except his name.
So when will he retire?
"I will keep sharpening until I die," he says, before letting off his trademark booming cackle of a laugh.