Mich. Comp. Laws § 331.81

Sec. 51. The state covenants with the purchasers and all subsequent holders and transferees of notes and bonds issued pursuant to this act, in consideration of the acceptance of and payment for the notes and bonds, that the notes and bonds issued pursuant to this act and the income therefrom and all its fees, charges, gifts, grants, revenues, receipts, and other moneys received or to be received, pledged to pay or secure the payment of such notes or bonds at all times shall be free and exempt from all state, city, county or other taxation provided by the laws of the state, except for estate, inheritance and gift taxes and taxes on transfers.