Hello, linguist here! This is absolutely a thing, although it's not quite a 'k' sound as you're hearing it.

Although 'ng' is written as two letters, it's just one sound - /ŋ/. This is a continuous sound, so you can basically hold it indefinitely. In most English accents, when you finish making the sound it just stops. However, in some accents there's a sound when it releases which is what you're hearing as /k/. The sounds /ŋ/, /k/, and /g/ are all pronounced in the same place in the mouth, so although we'd normally write the combination as /ŋg/ your interpretation is totally valid since the voicing (vocal cord engagement) doesn't always stop at exactly the same time as the sound and that's the difference between /g/ and /k/.

It's one of those things that a lot of people do without even realising it. A good example is comparing the words 'singer' and 'linger' - someone who doesn't do this might still have the /ŋg/ sound in 'singer' because there's a boundary between a whole word and a suffix, but they'd definitely just have /ŋ/ in 'linger' because it's a whole word. Someone who does do this would always have /ŋg/ in both words - and in my experience would struggle to just say /ŋ/ on its own!

Last time I looked at the data this was mostly a thing in the Midlands and some parts of the North, with the huge caveat that language change is rapid so it might appear in more places now! This is also focused on England specifically because other varieties of British English are not my specialty.