The rate of seriously delinquent mortgages held by Freddie Mac inched higher in September to 3.51% from 3.49% in August, the government-sponsored enterprise said Wednesday. In the same month, the aggregate unpaid principal balance of mortgage-related investments at the GSE fell to $1.4 billion, and the single-family loan guarantee volume reached $19.5 billion in September, making up 60% of its purchase and issuance portfolio. Overall, the GSE’s mortgage-related securities and guarantees fell at an annualized rate of 2.6%, according to its monthly summary report. Freddie Mac modified 6,465 loans in September and 90,126 through the first nine months of the year. The company ended September with a total mortgage portfolio balance of $2.11 trillion, down from $2.19 trillion a year earlier. Write to Kerri Panchuk.