The SPY's volume has been declining since December 11. We've seen lower highs and lower lows.
The QQQQ's volume has declined since December 18. Also note the QQQQs are bumping into the 20-day simple moving average.
The end of December is essentially a Wall Street holiday, so declining volume could simply be a sign of early vacations and nothing more. But declining volume can also be a sign a waning buying interest.