We have a musty smell under the kitchen sink which has shown up after 40 years. There are no leaks in the piping and no smell from the drains. Any ideas?

"Musty" means there is a presence of water somewhere. You could have small drip inside the wall or perhaps some item stored under the sink that smells like wet wood.

Take paper towel and wipe off drain pipes and water supply with it. See if it is wet. At the same time, you can wipe off bottom of the cabinet as well...

Let me know what you found out... Back to you... Milo