I'd be surprised if the drive weren't on some sort of anti-vibration mounts; however it's not going to stop a noisy drive.
Thanks Galatic for confirming your drive to be 1TB, I'm guessing Stewpy and Beej78 also have 1TB models and not the 500GB.
The drive is spinning and recording all the time when the Humax is on, the heads will be postioned over the section where the 2hr chase-play/buffer is on the disk enabling the pause/rewind function. When a scheduled recording starts the heads have to move backwards and forwards from the buffer section to the new recording section many times a second.
Some drives are simply noiser than others and there'll be clackety-clack or clickity-click noise as the heads constantly move to reach different sections on the drive. Anti-vibration mounts, blu-tack or similar measures may help (worth a try before condemning), but will be of limited benefit as they mainly help against the constant whirr or drone vibration noise of the drive motor.
As Faust points out, noise is very personal and it's impossible at lower levels to say whether it's excessive or something to worry about or not. If it can be heard over the TV then I'd be worried and investigate further.
The original poster (OP) mentioned a noise every 10 seconds on average and the replacement box does exactly the same. Humax will be getting drives in different batches from several manufacturers and even the same drive will have different firmware over time and hence operate differently.
It's possible that the current batch of drives has a 10 second head auto-park function (a bit like lifting the needle/arm off a record/lp/vinyl whilst it's playing and putting it back on its rest/stationary postition every 10 seconds before moving it back to play again) set in its firmware.
On a pc, laptop or similar it would be dead easy to confirm this however; Humax have locked down the boxes making it impossible to check even the most simplest of things.
If the OP can confirm a regular 10 second or so hard drive noise that sounds like the heads are being parked then I'd suggest contacting Humax CS as it would seem to be a hard-drive firmware (not Humax firmware) issue that will prematurely wear the drive out.