Chief
Certainly not SOPs.
Plenty of senarios available but i'd have to guess that, to have the speedbrakes out for so long, they may have been given a late descent clearance coupled with a speed reduction for separation.
With a late descent clearance we'll be above the normal descent profile (thrust idle, 280-300kts).
The preferred way to regain profile is to increase our descent speed (to say 320kts) which means more drag and therefore a steeper descent angle allowing us to intercept the original profile. This is why you'll sometimes hear us requesting cancellation of the 250kts below 10000' speed restriction if we've been held high. (other times we just want to get there quicker
)
But to be given a speed reduction to say 250kts from ToD means we'll be gliding further as we're then slowing towards our best glide speed (~200kts clean) which will cause the descent angle to shallow and the acft to diverge even higher from profile.
The only option left (short of radar vectors for more track miles-last option) is to deploy the speedbrakes- more drag, steeper descent profile. But, because we're descending at a slower speed the speedbrakes are less effective (remember parasite drag curve from your PPL) so they must stay out longer. The rumbling and rougher ride you felt is why we prefer not to use them if possible- passenger comfort.
The change of sound you mentioned is probably when we're configuring the acft for landing- slats/flaps/gear and the engines spooling up from the idle descent to approach power setting. This starts ~5000' when we have to shallow our descent so as to slow to initial flap extension speeds.
Which leads me into something you might reference for your ATC'ing- most jet acft can't "go down AND slow down". If you want us to descend quicker- allow us to speed up, if you want us to slow down- expect our rate of descent to decrease.
I'm sure i've explained alot that you already know- but i hope this helps anyway. Thanks for the question.
HJ