I gather from what you've said that your line is noisy with both iplates, but was unsure whether you meant that it was OK without them?
My advice would be to test it as methodically as possible, starting from top...
1. Using a "known good" corded phone, test with iplate and modem (plugged in iplate). If noisey could be any number of things which need to be eliminated. So continue to step 2.
2. As above, but without modem plugged in. If not noisey, could be modem (or lead), or a line/exchange fault. If still noisey then looking like iplate/line. So, eliminate iplate.
3. Remove iplate and plug phone direct into test port. If still noisey in BT test port, then it's the line, unless you have extensions which you may or may not be aware of, piggy-backed off the NTE, known as star wiring.
Hope that helps get you started.
P.S. Consider the following too...
The type of noise. As a very, very general guide, hissing noises are often broadnad or filter related. Crackling noises tend to be line related, e.g. high resistance etc.
Are you using a cordless or corded phone. Some cordless interfere with ADSL.
Good luck, let the forum know how you get on