"background level" is correct and usable in written English.
You can use it when referring to the level of something in the background, for example, "The background level of noise was low."
Dotted line marks the detector background level.
No significant difference in background level was observed.
(there is no support for including a background level).
The fall-out reaching Scandinavia was only twice the natural background level of radiation.
But he does expect government to provide him with a background level of order.
Eberhard PH. Background level and counter efficiencies required for a loophole-free Einstein-Podolsky-Rosen experiment.
This ranking provides a background level against which to judge the rate of messaging during games.
Data were processed by color balance adjustment, background level correction, and quantile normalization.
Since then, the scientists refined their measurements, detecting a background level of 0.7 parts per billion.
It revealed a low background level of methane which spiked over 60 Martian days.
The presence of tumor is confirmed when the bioluminescent signal overcomes the background level.
