Amalek's contamination penetrates the camp

וישמע יהושע כו' קול מלחמה במחנה.

"And Yehoshua heard… the sound of war in the camp" (Shemos 32:17).

As Moshe Rabbeinu descended from Har Sinai, Yehoshua heard a tumult and took it to be the noise of battle within the Israelite camp.

פרשנו כי יהושע שהי' הנלחם עם עמלק.

We have explained that Yehoshua, who was the one who waged war against Amalek.

The Sefas Emes recalls that it was Yehoshua who had earlier led the fight against Amalek.

הרגיש שמעמלק בא החטא.

He sensed that the sin came from Amalek.

Because of his prior battle, Yehoshua had a special sensitivity to detect the influence of Amalek — and he perceived that the chet ha'egel (sin of the Golden Calf) had its root in Amalek's spiritual contamination.

רק מקודם כתיב כל הנחשלים שפלטן הענן.

But beforehand it is written that all "the stragglers" — those whom the Cloud had cast out.

The Torah relates that Amalek attacked "the stragglers" (Devarim 25:18), understood as those weak ones whom the Ananei HaKavod (Clouds of Glory) had expelled from the camp because of their unworthiness.

אבל לא הי' בו שליטה במחנה.

But it had no dominion within the camp itself.

Until now Amalek's corrupting power could only reach those outside, cast beyond the protective Cloud; it had no grip on those who remained inside the holy camp.

ועל זה תמה קול מלחמה במחנה.

About this he wondered: "the sound of war in the camp."

Yehoshua was astonished that Amalek's destructive influence — the "war" — could now be heard within the camp itself, where it had never before been able to penetrate.

שזה הצרעת נספח גם לתוך המחנה:

For this "tzaraas" had now attached itself even within the camp.

The spiritual affliction of Amalek, like tzaraas (a defiling skin-malady), had spread inward — the sin of the Calf showed that the contamination had breached the camp's holiness from within.

Summary: Yehoshua, having fought Amalek, was attuned to its spiritual influence and recognized that the sin of the Golden Calf stemmed from Amalek. Whereas Amalek had previously only been able to harm the "stragglers" cast outside the Clouds of Glory, Yehoshua was astonished to hear the "sound of war" within the camp itself — a sign that this defiling contamination had now penetrated inside.