If its a lot on the upstream get in the test socket new cable filter and do a download and see if they go up at the same rate
Line errors (FEC, ES, CRC) have absolutely zero correlation to data being transferred (doing a download as you put it).
That's not how it works.
You will get the exact same numbers of FEC with zero activity on the line as you would downloading/uploading at full speed for hours.
With how the frequencies work upload is next to the ones used by the voice so it normally affects each other.
On ADSL the entire upstream band sits next to the voice tones but on VDSL2 it's only a very small proportion of the upstream (US0).
There's also a guard band between the frequency/tones used by voice and that of the upstream, to prevent interference.
Tone 1 # POTs.
Tones 2-5 # guard band, prevents cross talk between POTs and DSL.
Tones 6-31 # Upstream band 0 (US0) on ECI cabinets
Tones 7-32 # Upstream band 0 (US0) on Huawei cabinets