This situation can be solved very easily...
HELLO!? IS ANYONE DOWN THERE!?
Haha, just kidding.
>Take point and advance quickly, but quietly to the source of the sounds.

Don't worry, I think yours was funnier.

Saber Squad, Descending To Level B
As you move down, there's a scuffle and sounds of running.

"They found their way down! Wea--"

"Shut up and hold your fire!"

"The ♥♥♥♥ are you on?"

"At ease, Sergeant."

"But..."

"I said, at ease, Sergeant."

"...sir."

You're almost blinded by a half-dozen flashlights strapped to guns poking from maintenance tunnel hatches and behind a makeshift barricade that spans almost the entire roadway, a barricade that seems to be made up of burnt out civilian cars and what looks like an old military truck.

The one apparently giving the orders - a man who looks to be in his late 30s, an ex-Terran Army veteran judging by the way he walks, the sort who retired and moved to the frontier to settle down - jumps down from the truck's old cab-top pintle-mounted gun, scoops an old K77 off one of the seats, and approaches slowly. The other figures stay in their hiding places, but lower their weapons and start to chatter amongst themselves nervously.

"Lieutenant Christopher Snipes, local Planetary Defense Force. What's left of it, anyway. Myself and the Sergeant are the only officers fit for duty right now. Good to see some friendlies..."

Saber Squad, Descending To Level B

"In short? Not good. They hit in the middle of the night, most of the militia squads barely had time to get their kit together before the fighting started. We rounded up everyone we could and hauled ass down here, but the network tunnels are so obscure a lot of people wouldn't know how to find us. There's probably only a couple hundred of the original colonists left at basecamp further up the tunnel, and we're low on food, water, and medical supplies. We'd been holding off skirmishers here for the last few days, but it seemed like they decided to focus elsewhere the moment you boys made planetfall.

"I have a hunch these boys aren't just here for human safari, though I'm pretty sure you ain't cleared to hear about it."

The truck is immobilized, surrounded on both sides by wrecked vehicles, and taking up almost the entire width of the tunnel. It'd take quite a feat to get it in a decent position to drive again, nevermind all the scorchmarks and bulletholes. The heavy gun works - but it's bolted firmly to the cab, and the configuration would make it hideously awkward to use without a bipod or tripod. Nevermind the weight, of course.

None of the militia have more than superficial injuries at this stage, although a couple of old wounds need redressing. They seem a little more confident after being handled by qualified medics with fresh supplies.

Dirk trades war stories with a couple of the older militiamen - ex-Terran Army troopers like Snipes. You sign up for some frontierworld to get away from it all, but they insist on throwing you back into the action... such is the life in the Corps.