The BT line test is intended to find a break in the line. Faults causing noise or crackling are rarely found by that test and the line usually teats OK.
So, firstly do the equipment checks to see if the crackling is caused by the BT line or by anything plugged into the line. The checks have been explained on this site several times previously but...
Unplug everything from the telephone sockets then plug a spare or a borrowed phone into the main socket (or the test socket behind the faceplate of the main socket if the bottom 2/3rds is removable).
If the line is quiet then something connected was causing the noise.
If the crackling is still present ring BT and book an appointment for an engineer.
You will be quoted the potential for a £99.00 visit charge should the fault be found on your own equipment - but you've already done the checks and found it's not that so you will be fine there. All faults on the BT network are fixed free of charge of course. No charge is raised if no fault is found either.
Once the fault is found and fixed it can take 48 hours or so for broadband speeds to return to normal.