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I’d agree with Kent. You may need some transformer isolation. I’ve seen comments elsewhere about Dell having a poor grounding design, at least on older laptops. I haven’t seen anything about whether they’ve gotten better on newer models. I have an older Inspiron 8200 that was bad this way. Even a high quality USB Pre interface had hum when recording into Audacity. Running on battery or (cough) lifting the ground on the Dell got rid of it. A stereo DI also fixed it. I also can’t recommend lifting the ground as it’s there for a reason. Mine uses a lump so its low voltage at the laptop as most are but the ground may offer static discharge protection for the laptop. Can’t say for sure though as I have a Thinkpad with only a two prong edison. Running on battery when recording would be the obvious cheap fix if the battery is good and has sufficient capacity for the needed time. You possibly could buy a high capacity battery for less than the DI or isolation transformers.