Two slightly noisy radiators

[chriselevate]

Hi, I've been doing work to a couple of radiators downstairs and had to drain my pressurised system down a couple of times. It's always filled up easily and after venting all the all out of the rad's it's working fine other a small "tapping noise" on one of the downstairs radiators and a "flowing water noise" from a heated towel rail upstairs. Both when the heating system is on.

Any idea why I'm getting this? I just want to quieten down the system as these two rooms the noise is quite noticeable.

Thanks.