The "social security" trust fund? You mean that little bag Santa Clause carries around on his back? There's no money there earning interest. There is an IOU with another IOU on the interest in the "Social Security" trust fund.
The trust fund is effectively long-term US Treasury Bonds. They are non-marketable bonds, but they are probably even more secure than US Treasury Bonds.
And, they are being redeemed now. It started a couple of years ago (about 5 years earlier than expected). But, it's expected to be exhausted about 2033.
The redemption of the bonds are paid out of general revenue. Since we are still running about a $400B deficit, the net effect is the trust fund is being converted to US Treasury Bonds that are sold to private investors.