This looks like a round barrow because it straddles the county boundary, and the Hertfordshire side has been ploughed, but it's actually* a long barrow [*er, actually, see below]. I had a childhood friend from Pegsdon and I remember her mentioning the story that you could hear someone inside knocking. I was (pleasantly) scared to believe it but I wanted to believe it. I can't remember if she said she'd heard it herself but I'm sure we must have pressed her on the point..
In her book 'Albion' Jennifer Westwood agrees that there is an old man inside who knocks to be let out. She mentions the Herts Illus. Review for 1894, which says a British Chieftain is buried here with his treasure chest. From time to time he knocks thrice on it to make sure it's still there (I guess it's dark in there). The hill is/was also known as 'Money Knoll'.
Westwood also suggests the name could come from 'cnycyn' - the welsh for a bump or small hillock. It's a long way from Wales, mind.
In her later book, 'Lore of the Land' (2005) Westwood mentions the Hertfordshire folklorist W B Gerish, who heard from a Mr Aylott that it was a 'warrior in armour' knocking. Purple pasque flower grew (grows?) on the knoll, and this was said to grow only where Danish blood had been spilt.
The scheduled monument record calls this a bowl barrow, and there is another nearby on a similarly prominent point on the hills here, at Tingley Field Plantation. Knocking Knoll was partially excavated in 1856 by William Ransom from Hitchin, and pottery from the site is apparently held in the Hitchin Museum.