Ok. first let me acknowledge the bike I have is a POS. So those who want to call it out... I already know it. Planning on getting something better once the snow is gone.

In the meantime I have this Mongoose XR250 bike.

Looks like this one....

I am riding it indoors on a magnetic trainer..... the last couple of days it has been making this popping noise and I know its from the crank area but not sure what it is and if its something thats an easy fix or not.

I don't have any LBS near me... would have to drive to one in a different town.

Here is the sound: https://www.knackjack.com/bikepop.wav

It's impossible to say how difficult it might be to fix. Bike noises are notoriously difficult to track down, and impossible to diagnose via remote viewing. Your best bet is to take it to a shop and see if they can reproduce the noise and offer advice.

Do the easy things first, eliminate the pedals as a source of the noise. Borrow a set of pedals known to be quiet (from another bike, friend or whatever), clean and grease the pedal and crank threads, install and tighten firmly. See if this helps. If it does you are done, otherwise go on to the harder stuff. Also, make sure that the noise is not caused by the front derailleur cable end tapping the crank as it goes past.

check the crankarms make sure they tight to the axle . there should be no play in them . Also check the bottom bracket for any play if so adjust it . if it a seal unit replace it .

The sound sure doesn't sound to me like any arm/pedal/ring bolts one to me. It's deeper then what I would expect. Perhaps BB in shell or bearings. But I do agree that one starts with the easy and lowest cost possibilities first if unsure. Please report back.

BTW the noise could also be frame/suspension. Noises can travel in ways many don't expect. Andy.

There's a bit of what sounds like a spring resonance. Are you sure it's not related to frame/suspension flex as you load the crank?

Other than that I have no idea, except that I've never heard a crank related sound anywhere close to that.

Sounds like a loose bottom bracket. Mine was making popping noises.

My bike had a broken bearing race on one side of the BB, and it made a sound similar to yours. The cranks and BB seemed tight, so it puzzled me for a while until I finally had a chance to overhaul my BB. That's when I found the broken bearing race. YMMV. I finished my overhaul with all loose bearing so that problem won't happen again. This, of course, may not apply in your case if your BB uses a sealed cartridge.